We report the experimental study on mode instabilities (MI) in large mode area step-index fibers by testing a 30/400 mm step-index fiber in a single-pass co-pumped all-fiberized amplifier, delivering up to ~550 W of extracted output power without MI. The pump power is increased well above the MI threshold to study the temporal dynamics of MI in detail, which are characterized by using both high-speed camera measurement with ~2200 frames per second and photodiode traces. The experimental results are compared with the theoretical results. The MI frequency component is seen to appear on top of system noise, such as electric noise, which shows that system noise may influence the onset of MI. The beam quality of the fiber amplifier is measured, which is ~1.4 before the onset of MI, and degrades gradually to ~2.1 after the onset of MI.