Humans are constantly susceptible to cognitive errors and these create biases in their judgments. The main purpose of this paper is to determine how the sentiment of the small investors affects their decision making by examining their portfolio returns. The proposed model of this research uses the logistic regression analyses to examine this relationship. Investor's biases have been measured by means of a questionnaire comprising several items. As for the selected sample, it has been composed of 178 small investors actively trading on the Tunisian stock market. The findings show that the portfolio returns of the small investors were somehow influenced by behavioral biases and the results indicate that anchoring, familiarity, age and experience to be important contributory factors to the portfolio performance. This study has implications for the development of the portfolio performance of the individual investor. Also some psychological aspects of a theoretical nature could not be wholly approached in a complete empirical way. The paper pushes investors to make their financial decisions based on their financial capability and experience level and to avoid relying in their sentiment when making financial decision. This paper incites the government to establish training programmes aimed to develop the individual investor financial literacy and competency