Abstract: For multiple reaction systems of methanol synthesis in the presence of Cu-based catalyst, CO and CO2 hydrogenations are selected as key reactions. In this paper, a diffusion model for the key components CO and CO2 is presented for calculating the effectiveness factors of C301 Cu-based catalyst pellet in two parallel reactions; numerical solutions of the effectiveness factors ξco and ξco2 are obtained. Reaction characteristics of the catalyst pellet is discussed. Experimental values of ξco2 and ξco are obtained at 5MPa pressure in an internal recycle gradientless reactor in which the macro-reaction rates take into account only the intraparticle diffusion on φ5 × 5mm cylindrical pellet of C301 Cu-based catalysts. The average absolute deviations between model calculated values and experimental values for ξco and ξco2 are 9.9% and 23.9%, respectively. Compa-risions between model calculated values and experimental values of ξco and ξco2 indicated that the model is simple and is feasible for the calculation of the effectiveness factors of parallel reactions.