The authors propose a measuring method of friction in main journal bearings of an engine under construction or assembly. A crankshaft supported by main bearings in crankcase joined to a rotary encoder in one side and combined with an eccentric flywheel in the other side. A crankshaft was rotated by an electric motor and then separated from it. The angular acceleration is measured from the pulses of the rotary encoder. The moment of inertia of crankshaft is calculated from the angular acceleration and the known values of the eccentric flywheel, those are the moment of inertia and the center of gravity. Friction torque was measured with the moment of inertia of the crankshaft and angular deceleration after the crankshaft was rotated by the electric motor and then separated from it. The friction torque was measured in the different bearing numbers.