To study effects of 9 cis retinoic acid (9 cisRA) on biological characteristics of lung squamous cancer cells, cell growth, cell differentiation and apoptotic indexes were observed in 9 cisRA treated L 78 and A 2 lung squamous cancer cells with flow cytometry. The results showed that 9 cisRA exerted marked inhibitory effect on the growth of two lung squamous cancer cell lines, 9 cisRA also had significant inducing effect on the differentiation of L 78 and A 2 cell lines, whereas the percentages of apoptosis of two cell lines in the treatment group were significantly higher than the control group. We conclude that 9 cisRA could inhibit growth inhibition, induce cell differentiation and apoptosis of L 78 and A 2 lung squamous cancer cells