No-tillage ridge-cultivation is a prominent representative of the conservation tillage systems for paddy fields in South China. It is, therefore of some important practical significance to quantify whether the conservation tillage system is able to contribute to improving soil organic carbon (SOC) sequestration and keeping stable and high crop yields as well, in paddy fields. Ridge cultivation changes micro-topography of the surface soil in paddy fields. However, for long, in their studies on organic carbon accumulation in topsoil of cropland under ridge cultivation and under conventional tillage, the difference in micro-topography of the surface soil between the two different cultivation systems has been often neglected, which would inevitably affect accuracy of the comparison. Hence, a long-term field experiment was conducted to investigate effects of no-tillage ridge-cultivation on SOC accumulation in ridge soil and crop yields in paddy fields, taking into full account the effect of ridges on micro-topography of the tested paddy fields. The long-term field experiment, initiated in 1990 and located in the experimental farm of Southwest University in Chongqing city, China (30°26′ N, 106°26′ E), is designed to have four different cultivation treatments in the field experiment, that is, Treatment CP1 (conventional tillage under the rotation of rice and winter fallow), Treatment CP2 (conventional tillage under the rotation of rice and rape), Treatment RN1 (no-tillage ridge-cultivation under the rotation of rice and winter fallow), and Treatment RN2 (no-tillage ridge-cultivation under the rotation of rice and rape). In this study, all the ridges in each plot under ridge cultivation were considered as a whole when calculated and compared SOC accumulation with the topsoil layers in Treatments CP1 and CP2 (the soil layers in the control), and the comparison required that the soil layers in the control and the ridges should have the same soil volume or mass in their respective plots. As the topsoil layers in conventional tillage treatments (CP1 and CP2) and the ridges in ridge-cultivation treatments (RN1 and RN2) had the same soil volume in their respective plots, Treatment RN2 was obviously higher than Treatments CP1, CP2 and RN1 in SOC density (p<0.05), while Treatment RN1 did not differ much from Treatment CP1 in SOC density, but both were obviously higher than Treatment CP2 (p<0.05). When the topsoil layers in conventional tillage treatments (CP1 and CP2) and the ridges in ridge-cultivation treatment (RN1 or RN2) had the same soil mass in their respective plots, SOC storage per unit mass of soil in the ridges and the topsoil layers displayed an order of RN2 > CP1 > RN1 > CP2, and the difference between the treatments was significant (p<0.05). All these indicate that Treatment RN2 is much higher than Treatments CP1, CP2 and RN1 in SOC accumulation efficiency in topsoil. Analysis of labile organic carbon (LOC) in the ridges and the soil layers in the control also indicates that Treatment RN2 was higher in LOC content and LOC/SOC than the others, revealing that Treatment RN2 provides better protection to LOC in SOC against mineralization, which is one of the main reasons why Treatment RN2 is higher in SOC accumulation in the ridges. In terms of their effects on yields of the crops, both Treatments RN1 and RN2 increased significantly rice yield (p<0.05), but Treatment RN2 did have some reverse effect on rape yield (p<0.05), which, however, didn't affect much the total economic output of the field from the two harvests (rice and rape) per year. In conclusion, Treatment RN2 is a kind of conservation tillage system combining environmental and economic benefits for paddy field, and worth extrapolating to the hilly and mountain areas of South China, where cold waterlogged paddy fields are extensively distributed.