Dynamic F0 contour plays an important role in recognizing speech. The present work examined the effect of F0 contour on speech intelligibility for hearing-impaired listeners for Mandarin Chinese in quiet, in steady noise, and in two-talker competing speech. The intelligibility of two types of natural speech was measured: single-Tone speech with relatively flat F0 contours and multi-Tone speech with time-varying F0 contours. The speech rate and mean F0 of speech materials were carefully controlled to avoid effects other than F0 contour on the speech intelligibility. Results showed that intelligibility was significantly higher for speech with a flat F0 contour than that with a dynamic F0 contour at a low signal-to-masker ratio in both speech-spectrum noise and two-talker masker.