Dugong (Dugong dugon) were surveyed using simultaneous visual and acoustical methods in January 2008 in Thailand water. A research boat monitored chirp and trill sounds produced by dugongs using a towed stereo hydrophone array system. Two teams of experienced visual observers conducted standard visual observation on the same boat. Comparing with two independent observations, detection probability of acoustic and visual monitoring could be calculated using Petersen method. The acoustic and visual detection probabilities were 8.7% and 9.3% . respectively.. The detection probability of acoustic monitoring raised up to 18.4% when conspecific chirp call was transmitted from an underwater speaker with the source level at 150 dB rms re luPa. In the mean time, the detection probability of visual observation was calculated as 13.2%. Vocal hot spots with many detection of calls were recognized. Dugong is one of the most difficult marine mammal species to observe visually by ship based survey. Passive acoustic monitoring will aid for the survey since it had similar detection performance with the experienced visual observers. Playback is suggested to increase the detection probability that is beneficial for future field surveys using passive acoustic method to make sure the attendance of dugong in focal area.