The forced inspiratory volume-time curve in 25 health subjects with normal ventilation function and 25 patients with COPD were determined, and MMIF, FIV were calculated from the curves. The PImax was measured in the same 50 subjects by modified Blac method sequentially. A close relationship between MMIF, FIV and PImax was found. This result indicated that MMIF and FIV could be indeed for evaluation of strength of respiratory, muscles. Additional observations of MMIF, FIV and PImax in 50 patients with COPD before and after inotropic drugs, aminophylline and CNB, further proved the validity of MMIF and FIV.