The fill charge and the sensitivity analysis of a micro-grooved heat pipe are presented. Methods for calculating the optimal-fill charge, the height and the temperature of the coolant liquid for a V-shaped microgrooved heat pipe are presented using a macroscopic approach. The fill charge is determined to be an important parameter for the optimal performance of a heat pipe. The under- and overcharged heat pipes are less efficient than the optimally charged one. The worsening effect of the overcharge is more pronounced than the undercharge. The variation in the coolant liquid temperature is greater than that of the substrate temperature. In the sensitivity analysis of a micro heat pipe, under transient operation, the variations in the thermophysical properties, gravity force, and design parameters are considered. The present study sets forth both the qualitative and the quantitative effects of the variation in thermophysical properties of the coolant liquid and the design parameters of a V-shaped groove. These studies are carried out by solving the coupled nonlinear governing partial differential equations for fluid flow, heat transfer, and mass transfer using the finite-difference technique. (c) 2006 American Institute of Chemical Engineers.