Electrical motors are one of the most important key components of industry. While motors can be divided in so many groups, mathematical description of all of them is divided into two subsystems: Electrical and mechanical subsystem. Mechanical subsystem is usually composed of rotor inertia, external load and friction in bearing of rotor. Friction is usually modeled as viscous friction, i.e. linearly dependent on angular speed. Rotor inertia J) and coefficient of viscous friction (B) are needed in order to model mechanical subsystem of motor. When motor is used in high performance close loop motion control systems, an accurate model of motor is required for system analysis and design. This paper suggests a novel method for measuring rotor's J and B for such applications. There is no restriction on the type of motor under test. Studied method, needs no sensor so no friction is added to motor. Only a digital camera is required. There is no need to open the motor case and remove motor's rotor in this method. Proposed method has been tested in laboratory and practical results shows effectiveness of suggested method. (C) 2017 The Authors. Published by IASE.