We have observed 85 UBV standard stars(V<6) with SNU photoelectric photometer system and 24" Cassegrain reflector at Sobaeksan Observing Station during the period between January and May, 1980 in order to examine the photometer system and to determine atmospheric extinction coefficients. From five night observations, the following results were obtained: (1) The set of filters(V=2mm GG 495, B=2mm GG 385+1mm BG 12, U=2mm UG 2) is better than the other filter set used in our tests on the basis of linear instrumental calibrations for V and B filters. (2) The SNU detecting system is very stable within ${sigma}=0.002$ in magnitude in a night but the variation in its stability from night to night is not negligible. (3) The extinction coefficients were derived from two different methods and we have found no systematic seasonal variation in the extinction coefficients although the mean dispersion is considerable.