Mode instability (MI) is an abrupt mode change when the average output power increases above a certain threshold power, which results in degrading beam quality and restricting the enhancement of power output for diffraction-limited high power fiber laser. The experimental study on MI in all-fiberized master oscillator power amplifer (MOPA) is presented in detail for the first time. It is revealed that MI in all-fiberized MOPA results in reducing the slope efficiency. The theoretical threshold power of MI for 20/400 step-index large-mode-area double-cladding Yb-doped fiber is about 1 kW and MI is rooted mainly from the thermal effect.