With the capability of plasticizing just the small amount of polymer melt required for micro part, polymer pellets plasticized by ultrasonic vibration energy is believed to be a promising alternative for traditional screw-barrel unit. Moreover, the enhanced melt flow properties and the lowered injection pressure and mold temperature all benefit the micro injection molding process in terms of production cost and energy consumption. Despite those advantages, there are still several challenges to be addressed to promote this new molding technology. In this talk, recent advances of micro injection molding based on ultrasonic plasticization concept are summarized in the aspects of the heat generation mechanism during ultrasonic plasticizing, the melt flow properties under micro scale, the design principles of the prototype machine and the characterization of the moldings. In the end, some future trends will be discussed.