With the successful launch of the first scientific satellite of the Republic of China (ROCAST-1) on 27 January 1999, Taiwan enters the era as a satellite data providing country. There are three scientific payloads on the satellite. One of them is the Ocean Color Imager (OCI). OCI is a push-broom reflective imager for monitoring ocean colors. OCI has taken images since March 1999. Because there is no on-board calibrator on OCI, a comparison with Sea-viewing Wide Field-of-view Sensor (Sea WiFS) data was performed in order to validate OCI data. Simultaneously in-situ observations of optical properties and chlorophyll a concentration were also collected in waters adjacent to Taiwan for vicarious calibration. We applied Sea WiFS atmospheric correction and bio-optical algorithms on OCI data to derive normalized water-leaving radiance and chlorophyll a concentration. Results show that the chlorophyll a concentration derived from OCI is generally larger than that derived from Sea WiFS. The correlation coefficient is about 0.60 with a root-mean-squared (RMS) of difference of chlorophyll a concentration of 0.10 gm/m3.