A sectional circulating assembly for a ball screw comprises an inner circulating block and an outer circulating block. In assembly, the inner circulating block is initially placed between the screw shaft and the nut before the rolling balls and the screw shaft are positioned, thus preventing the rolling balls from dropping out and the interference with the threads. After the rolling balls and the screw shaft are positioned, the outer circulating block is placed into the nut and to be engaged with the inner circulating block, and the outer circulating block is fixed to the nut. This method can maintain the tightness of the nut, thus preventing the leakage of noise.