Silicon-based electro-optic modulators contribute to easing the integration of high-speed and low-power consumption circuits for classical optical communications and data computations. Beyond the plasma dispersion modulation, an alternative solution in silicon is to exploit the DC Kerr effect, which generates an equivalent linear electro-optical effect enabled by applying a large DC electric field. Although some theoretical and experimental studies have shown its existence in silicon, limited contributions relative to plasma dispersion have been achieved in high-speed modulation so far. This paper presents high-speed optical modulation based on the DC Kerr effect in silicon PIN waveguides. The contributions of both plasma dispersion and Kerr effects have been analyzed in different waveguide configurations, and we demonstrated that the Kerr induced modulation is dominant when a high external DC electric field is applied in PIN waveguides. High-speed optical modulation response is analyzed, and eye diagrams up to 80 Gbit/s in NRZ format are obtained under a d.c. voltage of 30 V. This work paves the way to exploit the Kerr effect to generate high-speed Pockels-like optical modulation.