Pitch modification is an important part in text to speech synthesis and voice conversion. In voice conversion, many authors in favor of realizing the voice conversion by the method based on LPC analysis, with which the spectrum envelope is converted by modifying the LPC coefficients directly or indirectly and the pitch modification is realized by processing the LPC excitation spectrum or LPC residual. Frequency method is proposed to process the LPC residual, then the pitch modified LPC residual is got. This method can also be used in the pitch modification of the text to speech synthesis.