This study examines the behavioural and personality factors in the investment of public officials. Investment Decision Making (IDM) of the individual investors may be rational or irrational and here the study focused on the irrational IDM. The influence of eight types of behavioural biases and Big five personality traits (BFPT) on irrational IDM are studied along with mediation effect. Significant findings of the study are that BFPT and Behavioural biases pointedly influence public officials' Irrational IDM. Personality traits have a mediating role in behavioural bias and irrational decisions. The results suggest that personality traits and Behavioural biases are key in investment decisions and the investor education and awareness programmes are vital in overcoming the biases and lead to rational decisions.