Accidents are common in all sectors, but the construction sector has the largest number of accidents. As far as the construction industry in Thailand is concerned, the work at the construction site in Thailand is subjected to a high number of accidents due to high work-related risk rates. Thailand’s relatively enormous and stable economic growth has led to a rapid expansion in the influx of unskilled workers into the construction sector over the past few decades, especially in major cities. The objectives of this study are to develop a safety performance assessment tool and to collect data from medium and large construction projects based on a questionnaire tool that has been developed. The key performance indicators (KPI) were used to develop the performance assessment tool and questionnaire from the list of leading indicators. Lastly, the paper showed the result of the level of the safety performance of the construction companies in Thailand. Generally, three of the safety performance were in a poor level, namely preplanning risk assessment, emergency response system, and safety reward and incentive.