The antitumor effect and the mechanism of action of ursolic acid in A549 cells(human non-small-cell lung cancer cells) was investigated in this paper. Firstly, MTT assay was performed to test whether ursolic acid could inhibit the growth of A549 cells. Secondly, Western blot was utilized to measure the expression level of COX-2 and the activation of MAPKs. The MTI assay revealed that ursolic acid inhibited the growth of A549 cells. The result of Western blot suggested that ursolic acid inhibited the expression of COX-2 and activation of ERK and ERK specific inhibitor PD98059 suppressed the expression of COX-2 synergistically with ursolic acid. Taken together, our data suggest that ursolic acid can suppress LPS-induced COX-2 expression in A549 cells, which could be due to the inhibition of the activation of ERK.