Laser milling of micro-fluidic Grooves under vacuum condition is presented. The regularity of laser milling under vacuum condition is established according to the analysis of melting-material removal which provides a theoretical basis. Nd:YAG laser is used to mill micro-fluidic grooves of a kind steel of HPM75. The research outcomes show that when vacuum pressure is from -0.07 MPa to -0.5 MPa,the materials removal increases by 1.6% to 3.3%. Vacuum condition means that air pressure is reduced. When the parameters are preferable,the quality of groove is satisfied with width 0.225 mm and depth 0.058 mm,and materials removal increased,compared with common condition. Laser milling rate is higher and its surface quality is proved better than that of normal laser milling. It helps to control the precision of milling size,and it is an effective method to laser milling technology.