Beam structures are widely used in MEMS sensors and actuators. MEMS micro beams are usually curled due to residual stresses and the characteristics of micro beams subjected to both residual stress gradients and electrostatic forces must be investigated for providing accuracy information for designing sensors and actuators. In this work, a novel semi-analytical formulation to address the above needs is proposed. By assuming an admissible deformation shape and utilizing energy method to determine the coefficients of the shape functions, it is possible to find the pull-in characteristics of the curled cantilevers. Detail parametric studies are subsequently performed to quantify the influence of various geometry and processing parameters on the pull-in characteristics of those micro beams. The method and results presented in this work would be very useful for related micro sensors and actuator designs.