Abstract: We performed a systematic study of the superconducting transition temperature (T_c), critical current density (J_c), irreversibility field (H_irr) and flux pinning mechanism in MgB₂. The T_c and J_c are significantly decreased for low and high temperature and field, and the H_irr line is shifted to lower fields by the HP up to 1.2 GPa. Remarkably, we found that the hydrostatic pressure can induce a transition from the δT_c to the δ\ell pinning mechanism. The charge-carrier mean free path fluctuation, δ\ell pinning, is responsible for the pinning mechanism in MgB₂ at 1.2 GPa. According to the magnetic relaxation measurements, our results show that the pressure reduce the pinning energy and the critical current density, and increase the vortex creep rate. The size of flux bundles is almost constant.