A direct electro-optic sampling system is set up. A gain-switched InGaAsPinjection laser is used as the source of sampling and a sampler utilizing the longitudinalelectro-optic effect in GaAs substrate of microstrip line was built. The impluse responseof a InGaAs/InP photo-detector of 0. 35 mV/Hz1/2 sensitivity and 9 ps time reolution aredemonstrated.