Objective. To introduce the degree of complexity in characterizing heart rate variability (HRV), and to discuss the changes in complexity of the cardiovascular system during orthostatic standing posture. Method. ECG of 8 subjects were recorded during supine and orthostatic standing postures. Degree of complexity was used to analyze the HRV. Result. Compared to supine before orthostatic standing posture, R-R intervals and its standard deviation at 0-5 min, 5-10 min, 10-15 min and 15-20 min during orthostatic standing posture were decreased significantly; the degree of complexity of HRV at 0-5 min and 15-20 min during orthostatic standing posture were decreased significantly; the approximate entropy of 0-5 min and 15-20 min were decreased significantly. Conclusion. The results showed that HRV and complexity of cardiovascular system was decreased during orthostatic standing. It was feasible that the degree of complexity can be used to analyze HRV.