A method of manufacturing a door inner panel of a vehicle is provided to reduce the friction resistance during upward/downward operations of a window glass by matching a curvature of the door inner panel with a movement path of the window glass. A method of manufacturing a door inner panel of a vehicle comprises the step of manufacturing a melted mixture having a temperature of 180 to 230 degree by melting a mixture including 60 to 70 weight % of polypropylene and 30 to 40 weight % of a long glass fiber. The melted mixture is input into an injector including a mold for an inner panel such that the mold is charged with the melted mixture. CO2 having 0.1 to 2.0 weight % is input into the mold on the weight of the melted mixture charged into the mold. An inner panel material charged into the mold is cooled through coolant of a cooling part formed at an outer circumference of the mold.
