The purpose of study was to investigate the ultrastructural features of leukemic megakarocyte (LMK) in patients with acute megakaryocytic leukemia (M(7)). Analyzing the ultrastructure characteristics of LMK and positive ratio of platelet peroxides (PPO) in 11 patients with M(7) were analyzed on basis of transmission electron microscopic observation retrospectively. The results showed that the diameter of LMK in 7 out of 11 cases was less than 20 microm, in 2 cases of them, the LMK diameter was from 10 to 15 microm and their PPO positive ratio was more than 50%, most LMK displayed regular shape, less protrusions, irregular nucleus, high nuclear/cytoplasm ratio, tiny granules, undeveloped demarcation membrane system (DMS) and irregular tubules in cytoplasm; in 5 out of those 7 cases the diameter of LMK was about 20 microm, PPO positive cell count was from 8% to 22%, most showing round or horseshoe nuclei, more or less heterochromatin, no DMS and granules were found in LMK in 3 cases and 2 cases occasionally. In other 5 out of 11 cases, the diameter of LMK was from 20 to 40 microm and PPO positive ratio was from 16% to 80%, in which smaller LMKs were similar to those in former cases in shape, and the larger LMK had irregular protrusions, varied nuclear/cytoplasm ratio, more heterochromatin, prominent nucleolus, some of them contained developed DMS, tubules and alpha-granules. It is concluded that most patients with M(7) are predominant of LMK in stage-I and minority contained LMK in II or III stage simultaneously. The differentiation degrees of LMK are different in individual and various cases.