This research aimed to develop knowledge and awareness about environmental laws and participation in environmental conservation of probationers in MahaSarakham Province, Thailand. This study was divided into 3 stages. Stage 1: was the development of a training manual and construction of training evaluation instruments which consisted of a questionnaire on environmental laws, a questionnaire on awareness about environmental laws, and questionnaire about participation in environmental conservation; Stage 2: was the try-out of the training manual and the determination of the instrument qualities with 30 probationers; Stage 3: was the evaluation of the implementation of the developed training manual with 55 purposive and voluntary probationers for 3 days. The findings revealed that the training manual had an effectiveness index of 0.2889. The experimental group showed gains in knowledge and awareness about environmental laws and participation in environmental conservation from before participating in the training session (p <0.001). Also, they indicated more knowledge of environmental laws and participation in environmental conservation than the control group probationers (p<0.001). However, the two groups did not indicate different awareness about environmental laws (p = 0.146). In addition, the probationers with different ages did not show different knowledge, awareness and participation (p = 0.798). The statistical interaction of age and training experience were not found to be significant (p = 0.850).