Recently,ambient vibration test(AVT)is widely used tu estimate dynamic characteristics of large civil struc-tures.Dynamic characteristics ean be affected by various envirnnmental factors such as humidity,intensity of wind,andtemperature.Besides these environmental conditions,tire mass of vehicles may change the measured valnes when traffic-in-duced vibration is used as a source of AVT tor bridges.The effect of vehicle mass on dynamic characteristics is investigatedthrough traffic-induced vibration tests on three bridges;(1)three-span suspension bridge(128m+404m+128m),(2)five-span continuous steel box girder bridge(59m+3@ 95m+59m),(3)simply supported plate girder bridge(46m).Acceleration histories of each measurement location under normal traffic are recorded for 30 minutes at field.These recor-ded histories are divided into individual vibrations and are combined into two groups aceording to the level of vibration;oneby heavy vehicles such as trucks and buses and the other by light vehicles such as passenger cars.Separate processing ofthe two groups of signals shows that,for the middle and long-span bridges,the difference can be hardly detected,but,forthe short span bridges whose mass is relatively small,the measured natural frequencies can change up to 5.4%. Recently, ambient vibration test (AVT) is widely used tu estimation dynamic characteristics of large civil struc-tures. Dynamic characteristics ean be affected by various envirnmental factors such as humidity, intensity of wind, and temperature. Beside these environmental conditions, tire mass of vehicles may change the measured valnes when traffic-in-duced vibration is used as a source of AVT tor bridges. The effect of vehicle mass on dynamic characteristics is investigated through traffic-induced vibration tests on three bridges; (1) three-span suspension bridge 128m + 404m + 128m), (2) five-span continuous steel box girder bridge (59m + 3 @ 95m + 59m), (3) simply supported plate girder bridge (46m) .Acceleration histories of each measurement location under normal traffic are recorded for 30 minutes at field.These recor-ded histories are divided into individual vibrations and are combined into two groups aceording to the level of vibration; oneby heavy vehicles such as trucks and buses and the other by light vehi for the middle and long-span bridges, the difference can be detected, but, forthe short span bridges whose mass is relatively small, the measured natural frequencies can change up to 5.4%.