In order to design the relief system size of di-tert-butyl peroxide (DTBP) storage tanks, the runaway reaction of DTBP was simulated by accelerating rate calorimeter (ARC). The results indicated that under adiabatic conditions the initial exothermic temperature was 102.6 degrees C, the maximum self-heating rate was 3.095x10(7) degrees C-min(-1), the maximum self-heating temperature was 375.9 degrees C, and the pressure produced by unit mass was 4.512 MPa.g(-1). Judged by ARC test, the emergency relief system for DTBP was a hybrid system. Based on Design Institute for Emergency Relief System (DIERS) method, the releasing mass flow rate W was determined by Leung methods, and the mass velocity G was calculated by two modified Omega methods. The two relief sizes calculated by monograph Omega method and arithmetic Omega method are close, with only 0.63% relative error. The monograph Omega method is more convenient to apply.