Pitch is an important element of speech property. Pitch modification is necessary for many speech processing applications such as speech synthesis. Methods of pitch modification can be categorized as methods on time domain and methods on frequency domain. In this paper we propose a novel approach which can modify pitch on time domain. This approach takes advantage of human auditory perception property that human is not sensitive with the low energy parts of speech. Utilizing this property we shorten or expand the pitch period by deleting or copying the low energy parts of speech. Our experiments show that this approach can change pitch period effectively.