High aspect ratio structures are getting more and more attentions with the development of MEMS, MOEMS, bio-MEMS[1], etc. Today micro injection molding for thermoplastics is one most popular technology for the high aspect ratio micro structures[2,3]. However, due to the relative high viscosity of the polymer melts and the high adhesive force between the microstructure and the mold, ultra high aspect ratio micro structure are very difficult and expensive produced by traditional micro injection molding[4]. In this study, the UV curing polymer with extremely low viscosity combined with micro injection molding process was developed and applied for the high aspect ratio micro structure fabrication. With the presented UV assistant micro injection molding device, the lower viscosity of the UV curable resin make higher aspect ratio micro structures able to be formed in the cost efficient way. The main units and construction design of this UV assistant micro injection molding equipment are dedicated in detail and explained for its application for high aspect ratio micro structures fabrication.