In Vietnam, the quality of tuna caught by handlines with artificial lights seemed to have declined compared with those caught by longlines. In order to determine the main causes of such phenomenon, surveys were conducted by interviewing concerned fishers while four trials were also conducted on tuna fishing boats, namely: absolute blooding out of catch, changing lights power, changing lines hauling speed, and fishers' normal practice of handling (as control). Catch from the first three trials were compared with the control using chemical indicators as criteria, such as total protein and NH3 contents, protein oxygenation level, total acid and lactic acid contents, and amino acid level. The paper suggested to reduce line hauling speed, which may improve the quality of tuna caught by handlines with artificial light.