Abstract: Rough set theory researched problems of tentative.uncontinuity and uncomplete in the messages space of knowledge.And it belives that the knowledge appears in people's ability of classification.This paper mainly discussed the usape in rough set theory in semigroups.Based on the qpproxion ations comcept,it brought in the rough subest,rough Descarte's product, left(right, double-sides)rough ideal and bi-rough-ideal in the semigroups.It also analyzed some of its algebra structure qualitatively. Thus it enriched rough set theory in semigroups.