A motor mounting assembly of an electric bicycle has a motor-installation mount, a motor unit, and at least one adapting piece. The motor-installation mount has a mounting frame and a lateral cover. The mounting frame is annular and has a first lateral side, a second lateral side, and a lateral opening disposed on the first lateral side. The lateral cover is assembled to the first lateral side and covers the lateral opening. The motor unit is received within the motor-installation mount. Each one of the at least one adapting piece has two fastening portions. One of the fastening portions of each adapting piece is connected to the motor unit. The other fastening portion is clamped between and is connected to the lateral cover and the mounting frame.