A training wheel assembly for a bicycle includes an upright support bracket mounted on a frame, a first pivot arm pivotally mounted on the support bracket, a training wheel rotatably mounted on the first pivot arm, a second pivot arm pivotally mounted on the support bracket, and an adjusting mechanism mounted between the first pivot arm and the second pivot arm to adjust a movable allowance or relative position between the first pivot arm and the second pivot arm. Thus, each of the training wheels is movable upward to leave the ground or downward to contact the ground so that the user needs not to remove the training wheels from the frame of the bicycle when not in used, thereby facilitating the user operating the training wheel assembly.