Objective: To study the ultrastructure and biological characteristics of expansive type of hepatocellular carcinoma (EHCC). Methods:Examination of EHCC and infiltrative type of hepatocellular carcinoma (IHCC) (each 20 cases) by electron microscope (EM) to compare their ultrastructure. Results: The 40 cases were divided into 3 groups: 16 cases of well differentiated EIICC, 4 cases of poorly differentiated EHCC, and 20 cases of poorly differentiated IHCC. The ultrastructure of well differentiated EHCC was similar to the surrounding non-cancer hepatocytes; the characteristics of them were as follows: 1. Cell membrane was developed well and cell border was clear; 2. Round nucleus was of regular shape; nuclear membrane was smooth; 3.Nucleoli were round, regular and bigger than normal;and 4. Plentiful endoplasmic reticulum and mitochondria were well developed. The ultrastructure of poorly differentiated EHCC and IHCC were identical: 1.Membrane was poorly developed; 2. Irregular nuclei were deeply indented or iobulated and many pseudoinclusions were seen; 3. Majority of the nucleoli were big, sponges or ring-formed; 4. Organelles were plentiful or scanty and tended to be degenerated.Conclusion: Most of the EHCC were mature by EM observation; this explained the EHCCs slow growth pattern, but some still had invasive potential.