Background : Mucoepidermoid carcinoma is the most common malignant salivary gland tumor. Histopathologic grade of this tumor is the most important predictor of prognosis that has a great impact on treatment protocols. Tumor behavior is worse with aggressive outcome in high grade than in low-grade mucoepidermoid carcinoma, leading to a need for more intensive treatment. Methods: A retrospective clinical study and prospective review of histopathologic grading were done using the three most popular grading systems of 60 patients with mucoepidermoid carcinoma diagnosed at Surgical Pathology Department, National Cancer Institute (NCI) from 2005 to 2010. Results: Recurrence rate was strongly correlated with high tumor grade (P =0.003, 0.03 and 0.005 according to AFIP, Brandwein and Modified Healy grading systems). Male gender was significantly correlated with the tumor high grade (P =0.01). Lymph node status was significantly correlated with overall survival (P =0.026). Using Brandwein grading system showed that as histologic grade increased from low & intermediate to high, disease free survival (P =0.029) was significantly decreased. The Kaplan-Meier estimated 1-year, 3-year and 5-year overall survival were 65.2%, 55.5% and 41.0%; respectively. Conclusions: Outcomes of the cases with intermediate-grade mucoepidermoid carcinoma are less clear and can be categorized as low or high grade according to the used grading system. However, Brandwein grading system may have a better predictive value than the previously used Modified Healey and AFIP systems as it identifies very well the low grade cases. Further tools, such as immunostaining for Ki-67 in conjunction with tumor grade may predict the tumor prognosis accurately.