Cochlear implant is a surgically implanted electronic device that can provide a sense of sound to a person who is severely hard of hearing. The effect of a cochlear implant on residual, low frequency, hearing is complex and not well understood. This research focuses on changes of the cochlear mechanics due to a cochlear implant by comparing the basilar membrane, BM, response before and after the implantation using a computational model of the cochlea. In the model, cochlea implants were introduced into the lower cochlear fluid chamber and the active amplification process of the cochlear is not considered, since a passive cochlear model whose response does not depend on stimulus level can reasonably well represent the cochlea for subjects with hearing impairment. The results for the basilar membrane velocity show that the volume change in the fluid chambers due to the implant has a little effect, less than 3 dB at low frequencies, on the basilar membrane velocity. A more extreme condition, in which the cochlear implant is assumed to touch the basilar membrane at some or the whole positions and thus impeded its motion, was also studied. Although there is no travelling wave propagating in the basal region in the latter case, the remainder of the cochlea is still coupled to the stapes by the incompressible fluid. The basilar membrane velocity at low frequencies is relatively unaffected by the blocking of the basilar membrane motion in the basal region, although the effect is more dramatic for excitation frequency whose characteristic place is close to the end of the implant. Although this work does not model every aspect of the hearing loss after cochlear implantation as measured clinically, it does provide a way of predicting the possible mechanical effects of the implantation on the cochlear passive mechanics and residual hearing.