Field experiment and Laboratory study were made for the comparision of transformation of nitrogen and the yield of rice in the paddy soil ploughed under dry and water-logged conditions. The drying of the soil after ploughing did not effect the ammonia-nitrogen content of the soil or the yield of rice under field conditions. After water-logging, the amount of nitrate-nitrogen decreased rapidly, and it disappeared within a few days as the soil intensely reduced. The effect of drying of different typies of paddy soil on the ammonia-nitrogen content were studied in the Laboratory. These results showed that there was a Large increase of ammonia-nitrogen content in the soil under water-logged conditions after drying as compared with the non-air dried samples. The magnitude of the increase in ammonia-nitrogen content was influenced by the characteristics of the paddy soil, such as the redox potential and the content of organic matter, especially the degree of drying. It is considered that the drying of paddy soils under cultivation is a powerful means in increasing the soil fertility and the yield of rice, especially for the subhydrogenic paddy soils, but in case of soils with high redox potential and low content of easily decomposable organic nitrogeneous compounds, such as those soils derived from loess-like material, on which this field experiment was conducted, the drying of the soil did not increase the amount of ammonia-nitrogen of the soil or the yield of rice.