In this paper, we propose an approach for estimating pitch of speech in noisy environments based on instantaneous frequency(IF). First, we define the IF amplitude spectrum, which is obtained by projecting the STFT amplitude spectrum onto the IF axis. Based on the IF amplitude spectrum, we can perform harmonics enhancement by suppressing the aperiodic components. Next, we define an evaluation function to find pitch. This is done by expanding the IF amplitude spectrum to the tune region. Then we propose a method for obtaining a continuous pitch contour using the dynamic programming. Experiments show accuracy and robustness of our method especially when noise exists.