The influence of reaction parameters on the epoxidation of rapeseed oil (RO) with peracetic acid obtained in situ from the reaction between 30 wt% hydrogen peroxide and glacial acetic acid (AA) has been studied. The course of the reaction was measured by changes of the iodine number (IN) and epoxy number (EN), used to estimate the degree of rapeseed oil conversion, yield, and the selectivity of transformation to epoxidized rapeseed oil in relation to the total amount of oil undergoing the transformation. The optimal conditions of epoxidation are as follows: temperature 60 °C, molar ratio of hydrogen peroxide to rapeseed oil 9.5:1 mol/mol, molar ratio of acetic acid to rapeseed oil 1.12:1 mol/mol, stirring speed 500 rpm, and reaction time of 4 h. Under these conditions the epoxy number is equal to 0.157 mol/100 g RO and iodine number reaches low values of 0.123 mol/100 g RO. The selectivity of transformation to epoxidized RO calculated from EN and IN is 82.2%, conversion of hydrogen peroxide is 100%, conversion of RO calculated from IN is 60.8%, and yield of RO calculated from EN is 50%.