In past years, low-fat diets were recommended for the prevention and treatment of metabolic diseases without much attention to the quality of fat, but in current guidelines, the quality of fat is generally emphasized. Dietary fat intake and blood fatty acid (FA) compositions are associated with metabolic disorders including metabolic syndrome (MetS), diabetes and cardiovascular disease (CVD). FA compositions of phospholipids or cholesteryl esters in blood are also known to reflect dietary FA composition during the recent two months and the endogenous conversion of ingested FAs by desaturation and/or elongation. People with insulin resistance (IR) and MetS had high concentrations of palmitic acid (16:0) and dihomo-γ- linoleic acid (20:3n-6), and a low concentration of linoleic acid (18:2n-6) in their serum phospholipids. In addition, the desaturating enzyme activities, such as Δ-9-desaturase (18:1n-9/18:0 or 16:1n-7/16:0) and Δ-6-desaturase (18:3n-6/18:2n-6) were increased and Δ-5-desaturase (20:4n-6/20:3n-6) was decreased in obese or MetS individuals compared with healthy people. Convincing evidences reported that partial replacement of SFAs with PUFAs decreases the risk of CVD, especially in men. Beneficial effect of MUFAs on insulin sensitivity and insulin levels was considered as probable in comparisons of MUFA and carbohydrates versus SFA, whereas no effect was found on fasting glucose levels in these comparisons. Very recent data shows that higher total MUFAs, oleic acid (18:1n-9), palmitoleic acid (16:1n-7), and Δ-9-desaturase activity in blood phospholipids are associated with early alteration of fasting glycemic status, which can be useful markers for predicting the risk of type 2 diabetes (T2DM) and CVD. To elucidate the effects of the amount and quality of dietary fat on metabolic parameters (classical and noble new markers) reflecting the risk of T2DM and CVD, further evidences are needed to be supported by highly controlled randomized clinical trials and prospective studies with sufficient number of subjects and long enough duration.