The road traffic noise in Hanoi is characterized by a large number of motorbikes emitting frequent horn sounds. In order to cross-culturally investigate the effects of horn sounds on annoyance caused by road traffic noise, psychoacoustic experiments were conducted in Hanoi, Vietnam and Kumamoto, Japan. The annoyance caused by 12 types of road traffic noise with and without horn sounds was evaluated using two noise annoyance scales: an 11-point numeric scale and a 5-point verbal scale. The conditions and procedures of the experiments were the same in Hanoi and Kumamoto. The main findings are as follows: (1) the Vietnamese subjects were less affected by horn sounds than were the Japanese subjects; (2) the Japanese subjects were more annoyed by road traffic noise with horn sounds than that without horn sounds; (3) the Japanese subjects were generally more annoyed by road traffic noise than the Vietnamese subjects; and (4) differences between the characteristics of road traffic noise in Japan and Vietnam affected the annoyance of the Japanese subjects.