The photo-ionization effects in high gain intrinsic gallium arsenide (GaAs) photoconductive semiconductor switches (PCSS) are explored. In high gain GaAs PCSS, each stage in which the streamer is formed consists of photo-ionization and domain electron avalanche (DEA) and avalanche carrier growth. Photo-ionization effects create a local high carrier density region in which a local environment for the existence of the domain is provided. Photo-ionization effects include the laser trigger and recombination radiation originating from streamer. The optimum condition of optical trigger is discussed. It is computed that the recombination radiation produces excess carriers around streamer inside a local region about y≤30 μm and the product of average carrier density n(t=0) times characteristic length y is larger than 1012 cm -2, namely n(t=0)·y>1012 cm- 2. The characteristics of photoionization effects of streamer are investigated. The space threshold LEC (in the direction of electric field) of trigger region is found. The characteristic length LE of trigger region must be larger than LEC, namely LE>LEC.