It is important to understand the friction characteristics between piston-ring assembly and cylinder wall for the friction loss reduction as well as the solution of problem such as scuffing wear and oil consumption. A new system was developed for the piston-ring assembly friction force measurement. This system was applied to the friction force measurement to find its functional relationship with variables such as engine speed, oil viscosity, and engine load. The friction mean effective pressure(fmep) was found to have a linear relationship with$(vpsilon{U})^{0.42}$ under motering and with$(vpsilon{U})^{0.45}$ under firing operations, where $vpsilon$ is the kinematic oil viscosity and U is mean piston speed.