Objective. To investigate the effects of head down bed rest (HDBR), the simulated weightlessness, on the diastolic function of human left ventricle, and to discuss its role in cardiovascular deconditioning after space flight. Method. Six healthy young volunteers were subjected to -6 degrees HDBR for 21 d. Ultrasound Doppler technique was used to examine the changes of the diastolic function before, on the 10th, and 21st day during and 2nd day after HDBR. The orthostatic tolerance was also tested before and after HDBR. Result. Peak E-wave velocity (PEV) , peak A-wave velocity (PAV) , and velocity total integration of E-wave (VTI E), were significantly decreased (P<0.05) on the 10th and 21st day during and the 2nd day after HDBR, and velocity, total integration of A-wave (VTI A), ratio of E/A, and ratio of VTI E/A were also decreased, but did not reach the significant level (P>0.05). None of the six subjects passed the orthostatic tolerance test after HDBR. Conclusion. Simulated weightlessness can induce marked decline in diastolic function of human left ventricle.