Abstract: AIM:To investigate the effects of simulated microgravity on cell morphology, cell proliferation and cell cycle in human umbilical vein endothelial cells (HUVECs). METHODS: Clinostat was used to simulate microgravity conditions and cell morphological changes were measured using an inverted microscope. Cell proliferation was measured by cytometry and cell cycle alterations were examined by flow cytometer. RESULTS: Compared with those in the control group, HUVECs became round and grew in an overlapping manner in the clinorotation group. Cell numbers in clinorotation groups did not change significantly. Cell numbers in clinorotation group in G0+G1 phase at 12 h, 24 h and 36 h time-points increased significantly compared with those in controls (P<0.05). CONCLUSION: Simulated microgravity induces changes of cell morphology and inhibition of cell cycle in HUVECs.