OBJECTIVE: The paper applies the fractal dimension to heart rate variability (HRV), and discusses the changes of the fractal dimension of cardiovascular system during head down tilt (HDT). METHOD: ECG of 10 subjects were recorded during HDT, the experiment was repeated in each subject. Fractal dimension was used to analyze the HRV. RESULT: Compared to supine before HDT, the standard deviation of all normal to normal RR intervals (SDNN) at 15-20 min, 20-25 min, 25-30 min and restoration increased significantly; the root mean square of successive differences between adjacent normal cycles (rMSSD) at 20-25 min increased significantly; the fractal dimension at 0-5 min, 5-10 min, 10-15 min, 20-25 min, 25-30 min and restoration increased significantly. CONCLUSION: The result shows that HRV and the nonlinearity of cardiovascular system increase during HDT. It is feasible that the fractal dimension can be used for analyzing HRV.