To understand how frictional melting affects fault instability, we performed a series of high-velocity friction experiments on gabbro at slip rates of 0.85--1.49 m s-1, at normal stresses of 1.2--2.4 MPa and with displacements up to 124 m. Experiments have revealed two stages of slip weakening; one following the initial slip and the other immediately after the second peak friction. The first weakening is associated with flash heating, and the second weakening is due to the formation and growth of a molten layer along a simulated fault. The two stages of weakening are separated by a marked strengthening regime in which melt patches grow into a thin, continuous molten layer at the second peak friction. The frictional coefficient decays exponentially from 0.8--1.1 to 0.6 during the second weakening. The host rocks are separated completely by a molten layer during this weakening so that the shear resistance is determined by the gross viscosity and shear strain rate of the molten layer. Melt viscosity increases notably soon after a molten layer forms. However, a fault weakens despite the increase in melt viscosity, and the second weakening is caused by the growth of molten layer resulting in the reduction in shear strain rate of the molten layer. Very thin melt cannot be squeezed out easily from a fault zone so that the rate of melting would be the most critical factor in controlling the slip-weakening distance. Effect of frictional melting on fault motion can be predicted by solving a Stefan problem dealing with moving host rock/molten zone boundaries.