A pulse-width-modulation (PWM) control system with nonlinear ramp is disclosed. A nonlinear ramp generator generates a nonlinear ramp varied with the duty (V.sub.out/V.sub.in) in a waveform signal, which could be a logarithm ramp, an exponent ramp, a multi-piecewise-linear ramp, a power ramp or a combination of above. The slope of the ramp is not a constant due to the non-linear characteristic. The voltage V.sub.ramp will vary with the input voltage V.sub.in, output voltage V.sub.out, and duty (V.sub.out/V.sub.in), therefore it will reduce the influence of the input voltage V.sub.in or output voltage Vout on the modulation gain and loop gain, even to keep the modulation gain and loop gain in constant value. As mentioned-above, the present invention improves the transient response of system, the sensitivity for variation of V.sub.in and V.sub.out, thus it is capable of correcting the output voltage quickly, for supplying a more steady power output.