The negative tone electron beam resist of ZEN4100 whose base resin was polystyrene-copolymer has been developed for photomask fabrication. We have investigated how polydispersivity influenced to this resist. In this paper polydispersivity is an important parameter to determine polymer properties on contrast because the sensitivity of resist is a function of molecular weight which is deeply related to polydispersivity. For this experiment we prepared two sample polymers which were extracted from the original polymer. A weight of polymers was higher than that of the original polymer. Another molecular weight of polymers was lower than that of the original polymer. The polydispersivity of the both polymers were lower than that of the original polymer. When we applied sample polymers to the resist material, the contrast of the resist resulted in excellent. We found that the resist which was made from the low molecular weight polymer had good properties as the previous paper said. And against our expectation high molecular one had good properties in terms of gamma value. We have focused on this method for applying to photomask fabrication with a commercially available way.