A continuously tunable substrate integrated waveguide (SIW) phase shifter based on the buried varactors is proposed. A number of varactors buried within the substrate are introduced into the SIW section. Varactor diode is a nonlinear device, of which the capacitance varies with applied voltage of an exponent factor negative fractional value. The distribution of electric and magnetic fields can be changed with varied capacitance, which results in the phase-shifted. As one example, a phase shifter with four varactors is designed and implemented. Simulation and experimental results are in good agreements. The measured results show a return loss mostly better than 10 dB and an insertion loss less than 2.3 dB within a frequency range from 1.6 to 2.6 GHz (about 47.6% relative bandwidth). The maximum phase range is about 30° at the center frequency.