Objective To explore the inhibition mechanism and safety of pyrazolo[1,5-a]pyrazin-4(5H)-one derivatives against proliferation of human lung cancer A549 cells, H322 cells, and human umbilical vein endothelial cell(HUVEC). Methods Cells were treated with 40 μmol/L of the ppo3 a, ppo3 b, ppo3 i, and 0.1% DMSO(control) for 48 hours, respectively. Apoptosis was determined by Hoechst 33258 staining assay in H322 and A549 cells. Cell cycle distribution was determined by flow cytometry analysis in A549 cell. LC3-II, p53, and heat shock protein(HSP) 70 protein levels were detected by Western blotting in A549 cells treated with ppo3 b for 48 hours. The morphology and viability of HUVEC were observed by inverted microscope and sulforhodamine B(SRB) assay. Results Ppo3 a, ppo3 b, and ppo3 i significantly induced apoptosis in H322 and A549 cells. A strong G1-phase arrest was concomitant with the growth inhibitory effect on A549 cells. Ppo3 b effectively elevated the p53 protein level, but significantly reduced the HSP70 protein level. There were no significantly inhibitory effect on the morphology and viability of HUVEC when treated with ppo3 a, ppo3 b, and ppo3 i. Conclusions ppo3 a, ppo3 b, and ppo3 i could inhibit H322 proliferation through apoptosis and inhibit A549 through apoptosis and G1-phase arrest. The protein p53 and HSP70 might involve in the inhibition effects. These derivatives might be a clue to find effective and safe drug for lung cancers. Objective To explore the inhibition mechanism and safety of pyrazolo [1,5-a] pyrazin-4 (5H) -one derivatives against proliferation of human lung cancer A549 cells, H322 cells, and human umbilical vein endothelial cells were treated with 40 μmol / L of the ppo3 a, ppo3 b, ppo3 i, and 0.1% DMSO (control) for 48 hours, respectively. Apoptosis was determined by Hoechst 33258 staining assay in H322 and A549 cells. by flow cytometry analysis in A549 cells. LC3-II, p53, and heat shock protein (HSP) 70 protein levels were detected by Western blotting in A549 cells treated with ppo3 b for 48 hours. The morphology and viability of HUVEC were observed by inverted Apo G1-phase arrest was concomitant with the growth inhibitory effect on A549 cells. Ppo3 b effective elevated the p53 protein level, but signifi There were no significant inhibitory effect on the morphology and viability of HUVEC when treated with ppo3 a, ppo3 b, and ppo3 i. Conclusions ppo3 a, ppo3 b, and ppo3 i could inhibit H322 proliferation through apoptosis and inhibit A549 through apoptosis and G1-phase arrest. These proteins might be a clue to find effective and safe drug for lung cancers.