A new acid fracturing design model called double fluid system model is presented in this paper. In this model the difference in the ability to create fracture geometry between acid and pad fluids is considered. The fracture width during acid fluid injection period does not increase, nor remains constant as some papers stated, but decrease. This may be inportant because many design aspects, such as acid concentration along the fracture, effective fracture length and acid–etched fracture conductivity etc., depend on fracture width. According to this new idea the concentration in the fracture is lower, and the effective length is shorter because the fracture width is narrower than the data published previously.