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We present the modeling of passively Q-switched Er3+/Yb3+-codoped double-clad fiber lasers (EYDFL). Both two-level and three-level saturable absorbers (SAs) are used. The model is based on detailed rate equations of SAs, Er3+, and Yb3+ populations in the fiber, and light propagation equations including spontaneous emissions due to both Er3+ and Yb3+. Laser output characteristics including pulse energy, width, peak power, repetition frequency (PRF), and average power are computed as functions pump power, fiber length, single-mode/multi-mode fibers, Er3+/Yb3+ concentrations, out-coupling reflectivity, focus area on the SA, and SA doping and length. Several transition metal-doped crystal SAs and semiconductor SA mirrors (SESAM) are considered and compared.