To investigate the chaotic characteristic of respiratory sinus arrhythmia and nonlinearity in heart rate variability (HRV), two kinds of voluntary breathing, namely, paced breathing and breath-holding, were compared with spontaneous breathing. High frequency component (HF) and its band width of HRV were changed according to respiratory patterns. Correspondingly, reductions of noise limit, which has been documented as a sufficient proof of chaos, were shifted in both frequency and band width depended on HF peak by a new-established band- phase-randomized surrogate method in combination with numeric titration. These preliminary results demonstrated that the spectral component of respiratory sinus arrhythmia is likely a nonlinear index, rather than linear one, as conventionally accepted.