The relationships between the rates of starch synthesis and the activities of enzymes responsible for starch biosynthesis in developing grains of normal, pop, sweet and waxy corns were investigated and compared throughout the grain filling period. The results indicated that the rates of starch synthesis and the activities of sucrose synthase (SS), soluble starch synthase (SSS), granule-bound starch synthase (GBSS), starch-branching enzyme (SBE) and starch-debranching enzyme (DBE) each exhibited a single peak during grain filling period. Normal corn showed significantly higher SS activity than other genotypes between 30 and 40 DAP. The mean and maximum activities of SSS were in the following order: normal corn>waxy corn>pop corn>sweet corn. GBSS activities were significantly higher in normal corn, and significantly lower in waxy corn at late filling period. SBE activity of waxy corn was significantly higher than other lines after 10 DAP. DBE activity of sweet corn was extremely low and completely lost at 40 DAP. The rates of starch synthesis had some correlation with the activities of SS, SSS, GBSS and SBE during the grain filling process. No correlation was found between the rates of starch synthesis and the activities of ADP-glucose pyrophosphorylase (AGPase) and DBE. SS activity appears to play a major role in starch biosynthesis in maize. GBSS is responsible for amylose synthesis especially in the later period. SSS and SBE are associated with amylopectin biosynthesis.