A significant percentage of energy is lost in internal combustion engines due to valve train friction. The reduction of friction losses of a valve train is an important factor to improve significantly fuel efficiency and consequently emissions reduction. The friction loss of valve train includes the losses of the individual components and factors, such as spring load, temperature, lubricant type, mass of components, operating conditions and others. The breakdown of the friction within the valve train is necessary for the implementation of effective optimization measures.This paper presents an analysis of some influence factors associated to friction loss, utilizing the Design of Experiments approach. The results are based on dynamic measurements on a single overhead camshaft (OHC) engine valve train, performed on a test rig developed for friction torque measurements.