In the world over, the mainstream method of investigating coral reefs is conducted by scuba diving. Take the domestic physical examination of coral reefs for example, divers use the basic ¡§transect¡¨ method to estimate the coverage rate of coral reefs and to identify characteristics of the seabed environment. In addition, amount of specific fishes and invertebrates were recorded to gauge the influence of human activities exercised on coral reefs. Because the transect method can facilitate full and accurate investigation into the coral reefs in the surveyed area, it has been widely adopted by relevant domestic and international organizations for many years. However, in respect to charting out the area of coral reefs, scuba diving reveals some technological drawbacks, such as slower in investigation speed and lack of accurate positioning system. Also, this method imposes dangers on divers. One way to improve these deficiencies is by the application of sidescan sonar system, which renders investigation more speedy and positioning more accurate. This research attempted to use sidescan sonar technology to survey the coral reefs at two sites, i.e. the area of the Third Nuclear Power Plant and offshore area near Tiaoshi (the total surveyed area was about 0.94km2). In addition, optical images of the seabed, collected by a towed operated vehicle, were used to groundtruth the sonar imagery. As for investigation speed, an underwater terrain of 3,600m2 could be surveyed by the sidescan sonar system per minute, with scan range setting at 30m and ship speed of 2 knots. To enhance positioning accuracy of the sonar imagery, a DGPS was incorporated to the sidescan sonar system. The precision of the geocoded sonar imagery was within 5m. The major steps in this research were¡G 1. The surveyed seabed was classified into rocky and sandy seabeds based on the characteristics of sonographs. Furthermore, the geographic distributions of these seabeds were mapped. 2. Sonographs characteristics were groundtruthed by optical images. 3. By the specific characteristics of sonograph, the rocky seabed could be further classified into three subcategories, i.e. rocky seabed with high density, middle density, and low density of reef organisms. 4. The distribution of each type of seabed was further verified with optical images. 5. The method of applying sonographes to chart out conditions and distributions of coral reefs area was established. The feasibility of this acoustical method was then estimated. The results of this investigation included¡G 1. the site near the outfall of the Third Nuclear Power Plant The major seabed of this site was rocky seabed covered by reef organisms of middle density. The area of this kind of seabed was 228,617m2, with most of it located at depth shallower than 5m. The surveyed rocky seabed covered by reef organisms of high density was 125,838m2, with most of it located near Houbihu(shallower than 10m) and off Leidashi area. 2. the site near Tiaoshi The major seabed of this site was rocky seabed covered by reef organisms of high density. The area of this kind of seabed was 183,325m2. Most of this kind of seabed was located off Tiaoshi at depth shallower than 7m and also stretched between Tantzewan and Shiniuzai at depth shallower than 8m. The results of this research illustrated that sidescan sonar system is considerably effective in mapping the distribution of coral reefs and in estimating living conditions of the reef organisms.