A low jitter threeelectrode gas spark gap is studied. One of the important characteristics of the thre eelectrode gas spark gap is stability,which depends on gas type, pressure, electrode shape and material.With investigation, N２ is chosen, geometry of triggerelectrode is designed as diamond，mainelectrode and triggerelectr ode material is brass.Experimental results confirm the validity of the methode . Switch jitter which is less than 2ns has been achieved.