An effective and precision apparatus for rapid determination of optical reflective and transmissive spectra is described. By using a rapid rotating optical fiber to sample the incident and reflected or transmitted light beams respectively and feedback of the incident beam intensity to monitor the gain of the photomultiplier for keeping the incident photo-current under constant level, the measured signal gives the transmissivity or reflectivity directly. Besides, due to rapid scanning of the sampled signals the requirement 04 the steadiness of the light source is not critical. The relative accuracy of the measurements in the range of 410nm to 650 nm is less than 2% and the time for scanning the complete visible spectrum is 2 minutes.