【Objective】Fertilization greatly influences soil structure and nutrients accumulation in soil aggregates. However, it is still unclear how aggregate-associated potassium (K) changes in distribution in red soil. Therefore, effects of long-term fertilization on aggregate composition and fractions of K therein in red soil were investigated based on a long-term fertilization field experiment in the subtropical area of China, in an attempt to provide certain theoretical basis for amelioration of red soil and improvement of K fertilization efficiency in that region. 【Method】 From the long-term fertilization experiment at Qiyang County of Hunan Province, China (since 1990), five treatments were selected, including CK (no fertilizer), NP (application of chemical nitrogen and phosphorus in combination), NPK (application of NP and potassium in combination), NPKM (application of NPK plus pig manure), and NPKS (application of NPK plus return of half of the straw harvested). Undisturbed soil samples were collected separately from the 0～20 cm layers of the five treatment plots in a 26 year-old (1990—2016) field of the experiment after the crop of corn was harvested, for fractionation of soil aggregates and analysis of total K (TK), non-exchangeable K (NEK) and exchangeable K (EK) therein, Then relationships of the K in the aggregates of different fractions with crop K uptake were explored. 【Results】 Results show: (1) Among the treatments, Treatment NPKM was the highest in proportion of aggregates of >2 mm, 1～2 mm and 0.5～1 mm in particle size, but the lowest in proportion of aggregates of 0.053～0.25 mm and <0.053 mm. However, Treatment NPKS did not affect proportions of aggregates of >2 mm, 1～2 mm and 0.5～1 mm. (2) Compared with Treatment NPK, Treatment NPKM was 24.37%～49.04% and 82.02%～176.3%, higher respectively in NEK and EK content in soil aggregates. The contribution rate of the fraction (>0.5 mm) of aggregates to the bulk soil (CRAB) in TK, EK, and NEK increased by 6.25%～31.97%, 5.72%～43.16% and 6.33%～41.98%, respectively, while that of the fraction (<0.25 mm) decreased by 14.37%～21.44%, 17.04%～33.66% and 34.40%～43.84%, respectively in Treatment NPKM. (3) No significant difference was observed between Treatment NPKS and Treatment NPK in EK and NEK content in aggregates of the same fraction and in contribution rate of the EK and NEK in each fraction of aggregates to the total K in CRAB. And (4) Linear regression analysis shows that when the contribution rate of the fractions (>0.25 mm) of aggregates increased by 1%, K uptake of the crop increased by 9.92～11.89 kg hm^-2.【Conclusion】 Long-term application of chemical NPK fertilizers and pig manure in combination may significantly improve formation of soil aggregates in upland red soil and hence increases contribution rate of the aggregates to the bulk soil in K and consequently crop uptake of K.