The purpose of this study was to provide the evaluation data of daylighting and electric lighting performance when a dimming system was applied to VDT(Visual Display Terminal) office spaces. For the performance evaluation, Radiance program was used for simulations, and the mock-up room(3.6x7.8x2.8m) was used for the actual test. The results of this study were as follows; 1) When the simulation data using Radiance was compared with actual data, there was a 5.9％ difference. 2) The mock-up office space gained a sufficient workplace illuminance of 679 lux with the dimming control system. 3) Recorded power budget was about 6 W/m² in the mock-up office space, and about 50％ of lighting energy could be saved using the dimming control system.