This Bachelor of Science thesis consists of an efficiency evaluation and creation of atheoretical model for an Atlas Copcos epicyclic gearbox. The thesis starts with atheoretical chapter containing the fundamentals of epicyclic gearing and the build of thespecific gearbox investigated. The following chapter contains former testing and theoryof power losses in a gearbox. After the theory is explained the next chapter containstesting, compromises and assumptions during testing and also results from the testing.The next chapter explains the Matlab calculation program based on the theory andmeasurements. Finally the model is validated against reference a reference model andagainst real gearbox measurements followed by some closing conclusions. The finalmodel calculates the efficiency rather well and has a linear difference. The difference isstatic since some losses have been left out, it can be fixed with an empirical correctionfactor which corrects the values and predicts an acceptable efficiency. The correctionfactor for the surface roughness also decreases the difference.