In this paper, the experimental set-up used for measuring the light output noise of a laser diode has been established. A weak electrical noise and light output noise can be measured by using the cross-spectrum approach. Experimental results obtained for 30 Vgroove InP/InGaAsP lasers at frequency region from 1 Hz to 100 kHz demonstrated that the light output noise and electrical noise is correlative in the superradiation region, and its spectrum form is also similar. According to the mechanism of light excitation in LD, the light output noise has been discussed. The theoritical analysis shows that the light output noise is separated into two parts, one caused by carriers fluctuation is full correlative with electrical noise, another caused by light quantum fluctuation is uncorrelative with electrical noise.