The effect of dietary fat intake on human health has been a hot topic in public for the past four decades. You are what you eat. Dietary fat, as one of the three macronutrients, plays a critical role in health, both bad and good. Metabolic syndrome (MetS) is one of the common clinical disorder. The prevalence of MetS has steadily increased and is linked to a number of major chronic diseases. Intense research has produced massive findings with some pointing out to dramatic changes that may cause significant modification of what we eat, the type of food with various contents of fats and the percentage of all sorts of fats. This article reviews the research findings from animal models, mainly from cross-sectional, longitudinal and ultimately from clinical trials regarding the prevalence of MetS, the relationship between dietary fat intake and MetS, as well as physiological and clinical effects of dietary fat on MetS. The negative and positive effects of fat on waist circumference, serum levels of triacylglycerol and high-density lipoprotein cholesterol (HDL-c), blood pressure, and fasting blood sugar, were individually reviewed as well. Moreover, the effects of mono- and polyunsaturated fatty acids substitution of saturated fats (MUFA and PUFA) and trans unsaturated fatty acids on MetS are separately discussed. Finally, contradictory findings and challenges regarding the relationship between dietary fat intake and prevalence of MetS have been explained.