An all-optical tunable photonic bandgap microcavity made from a two-dimensional polystyrene photonic crystal is fabricated by focused ion beam etching. The pump and probe scheme is adopted to measure tunability based on the femtosecond optical Kerr effect. An ultrafast response time of less than 120 fs is achieved for the tunable photonic bandgap microcavity. The microcavity resonant wavelength shifts 3.1 nm under excitation of 9.4 GW/cm2 pump intensity, which is in agreement with the theoretical prediction.