The flyer plate is sheared off and accelerates through the barrel to the explosive pellet in the exploding foil initiators. So,the flyer shape is related to barrel diameter. Effect of barrel diameter on exploding current and flyer velocity of exploding foil transduction unit was investigated by experiment and simulation. The results show that exploding current is dependent upon barrel diameters under the same discharging and exploding foil parameters. But the flyer velocity decreases as increasing the barrel diameter because of the increased flyer area. Finally, the shearing function of barrel is eliminated, resulting of maximum flyer area (Smax).So,when the barrel diameter increases into a certain extent, the flyer velocity maintains. For the copper exploding foil with the dimension of 0.3 mm×0.3 mm×0.005 mm,Smax would be 0.32πmm2 corresponding to 0.60 mm barrel diameter.The investigating results provide design qualification for barrel diameters.