In　this　paper　based　on　[1]　we　go　further　into　the　study　of　chaotic　behaviour　of　the　forced　oscillator　containing　a　square　nonlinear　term　by　the　methods　of　multiple　scales　and　numerical　simulation.　Relation　between　the　chaotic　domain　and　principal　resonance　curve　is　discussed.　By　analyzing　the　stability　of　principal　resonance　curve　we　infer　that　chaotic　motion　would　occur　near　the　frequency　at　which　the　principal　resonance　curve　has　vertical　tangent.　Results　of　numerical　simulation　confirm　this　inference.　Thus　we　offer　an　effective　way　to　seek　the　chaotic　motion　of　the　systems　which　are　hard　to　be　investigated　by　Melnikov　method.　©　1995　Shanghai　University　of　Technology　(SUT).