This paper aims at examining the effect of employee diversity on organizational performance usinga case study National Audit Office of Morogoro in Tanzania. This study specifically intended to determine the effect of employee’s gender diversity on the organizational performance, to determine effect of employee’s education diversity on organization performance and to assess the effect of employee’s age diversity on the organization performance. The study adopted quantitative approach whereby 62 structured questionnaires were distributed to employees using stratified sampling design. The collected data were analyzed using quantitative and applied descriptive statistics. The findings revealed that gender differences have a positive effect on work performance among work groups, also age differences on work groups, it has been proven that here is a helpful result related with personality, position, knowledge and maturity among groups members of work, also the state of education has a positive effect and cognitive conflicts between work groups. The diversity of the workforce based on age, gender and education level has been well investigated and confirms that there is a positive effect brought by the diversity of the workforce on team performance. The study recommends that the management of National Audit Office of Tanzania should ensure that studies are carried out regularly to have new information about the diversity of the work team and its effects on productivity in order to take possible measures when it affects negatively