Type 2 diabetes mellitus (T2DM) has assessed as the substantial serious widespread diseases globally. The etiology of T2DM is related to irreparable risk features such as (genetic, generation) beside revocable ones (diet-behaviors, lifestyle and smoking habits). Furthermore, the diet show an imperative function in the rapidly rising incidence of T2DM especially in developing countries, but a very limited number of studies have considered to what extent healthy eating diet could reduce the risks of T2DM and be a part of treatment in T2DM patients. The impartial of this research is to inspect various studies, which considered diets as a risk factor to increase or reduce the incidence of T2DM, plus, find an association between diet and T2DM treatment by assessing the impact of diets on HbA1c or glucose levels in T2DM patients. Dietary assessment was measured by many different validate tools such as Food Frequency questionnaire (FFQ), 24h dietary recall, a questionnaire with a seven-day food record, self-administrated diet history questionnaire (DHQ). Several studies proved the positive role of healthy diets to reduce the risk of T2DM, but many of them are without strong shreds of evidence. However, other studies showed the risk of unhealthy diets on increased T2DM incidence. The accessible information is not adequate to classify precise dietary-components related to T2DM incidence in the different population. Moreover, the impact of diets on glucose or HbA1c was not strong enough, and this is not appropriate to prove diet importance. For this reason, more validated and dependable studies are essential and needed in this scientific area.