Investor Behaviour often deviates from logic and reason, and investors display many behaviour biases that influence their investment decision-making processes. What are the reasons investors behave as they do? A portfolio investor’s behaviour often deviates from logic and reason. Emotional processes, mental mistakes, and individual personality traits complicate tough investment decisions. Thus, investing is more than just analyzing numbers or figures and making decisions to buy and sell various assets and securities. A large part of investing involves individual Behaviour. Ignoring or failing to grasp this concept can have a detrimental influence on portfolio performance of the investors especially in the long run. Behavioural biases in investing encompass many types. For example, cognitive biases refer to tendencies to think and act in certain ways. A cognitive bias can be viewed as a rule of thumb or heuristic, which can lead to systematic deviations from a standard of rationality or good judgment. Some controversy still exists about whether some of these biases are truly irrational or whether they result in useful attitudes or Behaviour. Other biases are more emotional in nature. Understanding investor behaviour can inform investors about these biases and help them improve their decision-making processes in selecting investment services, products, and strategies. As a result of the financial crisis of 2007-2008, the discipline of psychology began to focus even more on the financial decision-making processes of individuals. Although theory may deem markets to be efficient, investor biases can explain a lot about why assets are often mispriced. This research work attempts to highlight the effects of the cognitive biases on portfolio investment decisions, and the relationship with portfolio investor’s personality traits. The pedagogical assumption underlying this focus is that once the portfolio investors are aware of this mental error, they will notice this in the arguments of others and be able to resist this bias, and that they will avoid making the errors themselves. This will lead to optimal portfolio performance in the long run. This research work concluded that there is direct correlation between extroversion and openness personality trait with hindsight bias and over confidence cognitive bias, between neuroticism personality trait and randomness bias, between escalation of commitment personality trait and the availability bias. Also, there is a reverse correlation between conscientiousness personality trait and the randomness bias, between openness personality trait and the availability cognitive bias.