The purpose of this study is to examine the factors related with un-Islamic behaviours of Muslim youth in risky groups of the three southern border provinces of Thailand. This study adopts the quantitative research approach. The data was collected from 1,800 participants who were selected by a multi-stage sampling technique. Data analysis using the R: a language and environment for statistical computing was employed using mean, standard deviation, Pearson's correlation and multiple linear regressions. The results showed that a lack of youth supervision by Muslim leaders, lack of participation in Islamic activities, lack of Islamic knowledge, and lack of Islamic upbringing were statistically significant related with the un-Islamic behaviours. The lack of youth supervision by Muslim leaders was clearly the most strongly related determinant in explaining the youths' un-Islamic behaviours (beta value = 0.32). Moreover, lack of Islamic upbringing, lack of Islamic knowledge, and lack of participation in Islamic activities also contributed to explain the variance in the youths' un-Islamic behaviours (beta values= 0.20, 0.11, and 0.10), respectively.