Assembly tasks are often performed by one robot with fixtures. This type of assembly system has low flexibility in terms of the variety of parts and the part-presentation the system can handle. This paper addresses assembly without fixtures using two-manipulator robot. An active method using force feedback is proposed for the peg-in-hole assembly in highly uncertain environment. Assembly states are defined as status having unique motion constraints and events are modeled as variation of the environmental force. The states are recognized through identification of the events using two 6-d. o. f. force/moment sensors. The proposed method is verified and evaluated by experiments with round peg-in-hole assembly.