With the development of sensors, a study on carbon nanotube composites (CNT) used as force sensing elements is presented in this paper, which consists of carbon nanotubes with polydimethylsiloxane (PDMS) as a matrix. Nanocomposites of carbon nanotube and polydimethylsiloxane, CNT-PDMS with different filler concentrations have been successfully prepared via ultrasonic and mixed method. With different density, the electrical characteristics change as a function of the strain. The piezo-resistance and piezo-capacitance properties of these composites have been studied in detail. In our experiment, the gauge factor has reached 40 for piezo-resistance and 70 for piezo-capacitance. It is shown that there is an effective and reliable way, which is to change the density of CNT-PDMS nanocomposites, to set the features to sensing strain and stress for resistance and capacitance of the composites. This nanomaterial has a decent potential in mechanical quantity sensors field.