A new polishing technique for three dimensional micro structures using magnetorheological (MR) fluid is presented. Among various fabrication technologies of micro devices, some processes such as electroplating and wet etching which are widely used, give usually rough surface. However, there has been no polishing technique useful to micro structures, while various existing polishing methods are applicable only to flat surfaces or macro scaled stmctures. In this study, three dimensional micro channel smctures generated by copper electroplating and silicon wet etching are polished efficiently using,MR fluid. As a result of polishing of both structures, the average surface roughness cuts down to more than an order with little change of original geometries and the performance of both structures is noticeably improved.