The characteristics of the Effective Transission-Absorptance factor for evacuated tubular collectors with three types of back reflectors are presented. The problems how to reduce the computational time, how to simplify the program and how to calculate the incidence angle modifier are discussed. The results show that the performance of evacuated tubular collectors is subject to huge variation with the position of sun and the distribution of the diffuse radiation intensity on the sky. The performance of the evacuated collector with a diffuse reflector is higher than it with mirror reflector at noon. However The both performances are almost equal when they are evaluated as the all-day performance. The danger of evaluation by short time testing was indicated. Testing throughout several days, at least a day, may be recommended.