Biofouling of ships leads to increasing fuel consumption and may possibly cause translocation of non-indigenous species to another marine environment where they may cause significant impact on the local ecosystem. Anti-fouling paints play an important role in the prevention of biofouling in ships. In this regard, the authors have already conducted efficacy tests of anti-fouling paints using mussels and barnacles. The authors focused on using algae as the test organism in a bioassay. The laboratory bioassay was designed and its applicability for testing the efficacy of antifouling paints was investigated. Five types of antifouling paints with hydration type coatings containing 0, 5, 10, 20 and 40 wt.% of Cu2O were prepared as test paints. PVC plates used in the laboratory bioassay were 50 mm x 50 mm x 2 mm in size. The test plates were coated on one side with the test paint. The panels were aged using a dynamic rotating device under controlled condition with light shielding (water temperature: 20 ºC, rotation speed: 10 knots, period: 45 days) prior to the bioassay. The filamentous brown alga, E. siliculosus, which is one of the most common fouling macroalgae in ships, was provided by Kobe University Macroalgal Culture Collection. An algal suspension of 10 ml (ca. 1mg as algal dry weight) was filtered through a nitro cellulose membrane filter (f=47 mm, pore size 8.0 mm) under vacuum filtration. The membrane filter with algae was cultured in PESI medium (temperature: 20 ºC, light cycle: 12 h. light with irradiance of 20 mmol photons mmol/m2/s / 12 h. dark, period: 5 days) in a still water condition. After culturing the algae, the surface of the membrane with algae was affixed to the test and control panels. The laboratory bioassay with a flow-through system was conducted under a controlled condition (flow rate: 0.66 turnover/h, water temperature: 20 ºC, light condition was same as with the culturing condition, period: 1 day), using boiled seawater as the test medium. The efficacy of antifouling paints was evaluated by the parameters (e.g., L*) of E. siliculosus after the bioassay. The value of L* increased with increasing Cu2O content (Fig.1). A linear regression analysis was performed on the relationships between values of L* and survival rate of the algae. Results showed that the value of L* decreased with an increase of survival rate (Fig.2). The newly designed bioassay using brown algae under a flow-through system was successfully tested for screening anti-fouling paints. The assessment of the estimated paint performance from the bioassay was confirmed using correlation patterns between the value of CIELAB (L*, a*, b*) and the survival rate of the algae. This study also proved to be a significantly consistent method for assessing the effectiveness of existing or future antifouling paints.