Each and every person in the world has a different mindset about investing their money in different sources of investments. In this situation human mindset, the behavioural activities play an important part in the investment decision. This study going to examine one of the investment opportunities available for Jaffna District investors in the Colombo Stock Exchange (CSE). This study pursued to determine the impact levels of behavioural influences on the individual investor adoptions of securities at the Colombo Stock Exchange (CSE). It was directed by the main objective seeking to determine the impact level of behavioural factors on the individual investors’ investment decisions at CSE special reference to the Jaffna district. To meet the objectives of the study, Behavioural factors are considered as the independent variable with the proxies of regret aversion, overconfidence, availability bias, past trends of the stock, market information, buying and selling of other investors, and individual investors’ decision is the dependent variable. Primary data was collected using self-administrated questionnaires. It was based on the sample of 165 individuals selected through the CSE Jaffna district The data analysis for this study was performed with the help of SPSS. This study used multiple regression and correlation analysis. There is one main hypothesis and six sub-hypotheses were considered for this study. It was tested at a 5% significant level. The findings revealed that the overconfidence, availability bias, and past trends of stock have insignificant impacts on the investment decision of individual investors at CSE in the Jaffna district. The regret aversion, market information, and buying and selling of other investors have a significant impact on the investment decisions of individual investors at CSE in the Jaffna district. The ultimate finding of the study was that there is an ascetically significant impact of investors' behavioral factors influence on individual investors’ investment decisions at Jaffna District CSE Sri Lanka.