Abstract: Field experiment with different potassium levels was carried to study the effect of potassium on the microstructure and ultrastructure of storage root parenchyma cells,the respiratory rate and ATP content of storage root,the distribution of 14C photosynthates and the storage root yield in sweet potato (Ipomoea batatas(L)Lam cv. Xushu18).The results showed that the application of adequate potassium was favorable to promote the sink activity,for in the storage root of sweet potato the parenchyma cells which were perfect in membrane and rich in cytoplasm contained much more mitochondrion and plastid;the respiratory rate and ATP content were all increased.Adequate K application increased starch grains in a unit volume and starch content of storage root,accelerated the transportation of 14C photosynthates from leaf to storage root and increased its proportion in storage root,promoted the storage root growth rate and increased its fresh weight.All of those contribute to increase storage root yield.