Molecular mechanism of development and progression of gastric cancer which could be a base of molecular diagnosis was described. Amplification and point mutation of oncogenes are less common in gastric carcinomas, even though it is valuable for diagnosis. Amplification of ERBB2 seems to be an indicator for metastatic ability of gastric carcinoma. Overexpression of EGF/receptor system is a biologic marker for high malignancy. Diagnostic significance for scirrhous gastric carcinomas is found in over-expression of TGF beta, IGF and PDGF. Loss of heterozygosity on chromosomes 5q and 17p frequently occurs commonly in well differentiated type gastric cancer. More accumulation of molecular alterations in the development and progression of gastric cancer should make the molecular diagnosis more valuable in clinical field.