Purpose: To establish a simple mathematical model for investigating the mechanism of frequency components in cardiovascular variability. Methods: A close-loop model involved in hemodynamic and autonomic systems was developed. Three peaks of very low frequency(VLF), low frequency(LF) and high frequency(HF) were simulated successfully. The frequency of HF is determined by the ratio of respiratory frequency and heart rate. The results also indicated that nonadditivity of heart rate modulation by sympathetic and parasympathetic efferents may play an important role in mechanism of LF and VLF generation.