The electrical properties of GaN-based laser diode (LD) with 450nm wavelength were measured using the electrical derivative technology and a precis io n impedance analyzer.The sudden changes of the apparent characteristics including (IdV/dI)-I,Cp-I and G_p-I curves in the lasing threshold region were observed.However,in this region, its (IdV/dI)-I curve displays a sudden jump rather than a drop reported in previous referenc es.Correspondingly,the change trends of Cp-I and Gp-I curves in the threshold re gion are also obvious ly opposite to those in the narrow band-gap LDs.The same abnormal results between the wide-band Ga N based LD and the narrow band-gap LDs can be observed in the junction parameters extr acted accurately by our ac－IV method,namely,all junction parameter curves d isplay sudden changes in the threshold region but their change trends are also opposite to thos e in the narrow band-gap LDs reported in our previous papers.In the threshold region,the opti cal power increases by an order of magnitude,which could have an intimated relationship w ith the sudden increase of the junction voltage.All these abnormal electrical properties in w ide band-gap LDs conflict with the current lasing theory,so it will promote further impr ovement of the laser theory.