The invention provides a coupling assembly for connecting two shafts. A first shaft includes an outward flange on one end thereof with a plurality of first holes extending through the outward flange around an axis of the first shaft. A second shaft is a hollow shaft including an inward flange on one end thereof with a plurality of second holes extending through the inward flange with same diameter of the first holes and aligning with the respective first holes. The coupling assembly includes: a coupling bolt with thread on a first end and a second end thereof, which is inserted in the first hole and the second hole. A first nut is engaged with the first end of the coupling bolt. A second nut engaged with the second end of the coupling bolt. An expansion sleeve is with its inner hole tapering from one end to another end thereof. The expansion sleeve is interposed between the coupling bolt and the first and second hole to secure the coupling bolt relative to the first and second hole; the second nut is formed with one end thereof having a threaded hole to engage with the coupling bolt and with the other end closed, and a profile hole smaller than the threaded hole is centrally formed on the closed end. The coupling bolt is formed with a through hole along the central axis thereof to receive an assembly tool to tighten the second nut relative to the coupling bolt. The invention also relates to a method for coupling two shafts with the coupling assembly. With this invention, it is possible to couple two shafts with access to only one side of the shaft flanges.