AIM: To evaluate the expression of iNOS mRNA in different invasion ability colon carcinoma cell strains. METHODS: MTT was used to detect the growth and reproduction of colon cancer cell strain CW-2 and LS174T. RT-PCR and Northern blot were used to detect expression of iNOS mRNA in colon cancer. RESULTS: MTT growth curve displayed that colon cancer cell strain LS174T grew and reproduced faster than cell strain CW-2. RT-PCR showed that iNOS mRNA expressed strongly in CW-2 cell strain, while iNOS mRNA expressed weakly in LS174T cell strain. Northern blot detected that iNOS mRNA expressed obviously in CW-2 cell strain, but cell strain LS174T have no obvious iNOS mRNA expression. All-trans retinoic acid (ATRA) had no obvious effect on iNOS mRNA expression in CW-2 cell strain of colon cancer. CONCLUSIONS: ATRA has no obvious effect on iNOS mRNA expression. iNOS has a dual effect on tumor growth. In low-metastatic colon carcinoma CW-2, iNOS may exert a anti-tumor influence by cytotoxicity or inducing cell apoptosis. In high-metastatic colon cancer LS174T, iNOS produced low concentration of NO, which may be an important signal-transduction molecule for increasing blood supply and angiogenesis, which improve the growth, invasion and metastasis of tumor.