Ultrasonic fusion bonding is a potential method for mass-production of polymer microfluidic chips. However, the micro structures in the chips are easily damaged or even destroyed due to poor controllability and uniformity of the melted energy directors in ultrasonic bonding process. In this paper, a novel micro joint including energy directors and flow blocks is presented. The flow profile of the melted energy directors is controlled by the dimensions of the flow blocks, auxiliary energy-equilibrating structures are also designed to prevent the uneven melt of energy directors. Polymethyl methacrylate (PMMA) microfluidic chips with the new joints were fabricated and ultrasonic bonding experiments were conducted. Micro channels with characteristic dimension of tens of microns were successfully sealed in 0.3 s and free of deformation.