The objective of the study is to investigate the cause of double-decker accident by in-depth accident investigation method. The method could describe the critical conditions of the accident which include pre-crash during-crash and post-crash. Moreover the influencing factors of the accident can also be best explained by the method. In this study the rollover of double-decker accident is selected as a case study. The main reason is that in Thailand the double-deckers are usually used for sight-seeing purpose and there are also many reported severe accidents. In this study it is found that the factors of the accident consist of speed dimension of double-decker road condition and geometry of the accident place.