Obesity is one of the common health concerns among human beings. Excessive calorie intake through consumption of fats and carbohydrates has been evidenced to be the major factor in this regard. Those involved with obesity may also be suffered from health problems such as heart and cardiovascular diseases, colon cancer and diabetes type II Production of energy from fat is about twice the energy produced by carbohydrates. Therefore, in order to prevent the risk of obesity the amount of this substance should be reduced. Remove the lipid in the diet can lead to undesired functions in food along with appearing of undesirable sensory properties. In general replacements of fat are used to solve these kinds of problems. Lipid replacements based on carbohydrate in confectionary products are often suggested. Cake is one of the most favorite foods among people; therefore fat replacement can be used in the preparation of that. Flaxseed ( Linum usitatissimum L.) belongs to Linacea family and one of the herbal gum resources is the mucilage extracted from the seed. The gum can be used as fat replacer in different food products. Flaxseed mucilage was isolated by water and then sedimentated by ethanol, and finally dried. In order to produce low calorie cake with desirable features (properties), different variables (factors) including percentage of water, mucilage and fat ratio were considered. Finally optimization of the reaction and the optimum level of each variable were conducted by using response surface methodology (RSM).To evaluate the models selection and optimized formula, two responses, cake texture and batter viscosity were applied. These variables as the linear and quadratic were effective on the viscosity and texture, respectively; while there was an interaction effect between the water and mucilage contents. According to the results there was no significant difference between the measured - predicted viscosity and texture; and optimum conditions which were determined as 71.53% water, 1.60% mucilage and 10.38% fat. Physicochemical properties of batter and cake at the optimum conditions were also evaluated. It was observed that viscosity and specific gravity of optimized batter formulation is comparable with that of the control. Volume and moisture content of optimized cake was higher than those of the control. It was evidenced that weight loss of optimized cake was higher than control. There was no significant difference in firmness and color between optimized and control cake. In terms of sensory evaluation, it was determined that there was no significant different between optimized and controls (p?0.01). Generally results indicate that production of low-calorie cake by using flaxseed mucilage as fat replacer is possible, So that the lipid levels could reduce to 76.40%. Key words :Cake, Fat Replacer, Flaxseed Mucilage, Response Surface Methodology.