A frame of reference moving with constant velocity relative to an inertial frame is itself considered inertial. This is because an object maintaining constant velocity in one frame will also exhibit constant velocity in another frame moving at the same rate. The principle of relativity asserts that the laws of physics remain consistent across all inertial frames. Therefore, the motion observed in both frames will align, confirming the inertial nature of the moving frame. Understanding this concept is crucial for analyzing motion in different reference frames.