In this paper, an integrated transformer design methodology for dual-active-bridge (DAB) converters, which basically utilize leakage inductance so as to substitute a cumbersome inductor in the conventional DAB converters, is newly proposed. The proposed integrated transformer with integrated leakage inductance inherently includes leakage inductance and magnetizing inductance, whose values can be appropriately selected by modulating the number of turns and the center-core air-gap. Based on the theoretical analysis of the DAB converters, the major parameters, i.e., primary and secondary current levels and its phase difference, can be derived, and these values are utilized for magnetic analysis of the proposed integrated transformer. Considering core loss and copper loss of the proposed integrated transformer in the DAB converters, optimal values of the center-core air-gap <inline-formula> <tex-math notation="LaTeX">$l_{g}$ </tex-math></inline-formula> and the optimal number of turns <inline-formula> <tex-math notation="LaTeX">$N_{\mathrm {1,}op} ~\&~ N_{\mathrm {2,}op}$ </tex-math></inline-formula> can be determined so that high power efficiency is achieved at a normal operating point of the DAB converters. The 20W prototypes of the DAB converter with EER2834 and EER3019 of the ferrite cores were fabricated and verified by simulation and experiment. The design results showed that the optimal number of turns at <inline-formula> <tex-math notation="LaTeX">$l_{g} =2.0$ </tex-math></inline-formula>mm are found to be 11 and 10, for the EER2834 and EER3019, respectively. Compared to 94.5% and 96.3% of AC efficiencies for the conventional transformer and inductor set, the proposed integrated transformer achieved 95.8% and 96.9% of AC efficiencies at maximum power transfer point of <inline-formula> <tex-math notation="LaTeX">$\theta _{p} = \pi $ </tex-math></inline-formula>/2 for EER2834 and EER3019, respectively.