Optical properties were studied in the costal waters of the southern part of Korea based on twenty oceanographic stations from 1 st to 9 th February, 1977. Submarine daylight intensity was measured by using an underwater luxmeter (Toshiba No.9). Daylight intensity in the upper 30m depth layer was measured at 1 m depth interval. The absorption coefficient of the sea water in the area ranged from 0.101 to 2.539 (mean 0.578). The Secchi-disc depth in the area ranged from 0.8 to 13 meters (mean 5.33 meters). The relationship between absorption coefficient (K) and transparency (D) was k= 1. 704/ D. The mean water color in the area was 5. 75 (3-9) in Forel scales. The rates of light penetration for daylight at 1m layer in the area ranged from 13.18 to 82. 05% and the mean was 59.56%, while the rate at 5m layer ranged from 0.007 to 46.1% and the mean was 18.47%.