The piezoresistance and its time dependence of conductor-filled polymer composites have been investigated. To reveal the origin of the time dependence of piezoresistance, the creep of the polymer matrix is also studied. Based on the interparticle separation change under the applied stress, a model has been developed to predict the piezoresistance and its time dependence. By analyzing this model, the influences of applied stress, filler particle diameter, filler volume fraction, matrix compressive modulus, potential barrier height, and the matrix creep behavior on the piezoresistance and its time dependence are interpreted quantitatively. These predicted results are compared with the experimental data obtained on the polymer composites filled with conductor fillers, and good agreements were obtained.