In this paper, a microstrip-fed narrow slot antenna with end-fire radiation and expanded bandwidth under dual-resonant modes operation is proposed. At first, the open-ended slot radiator is studied and used to effectively suppress all the undesired even-order modes, reduce the antenna size, and form up the end-fire radiation pattern simultaneously. After that, by symmetrically introducing a pair of slot stubs near the electric-field distributions nodal lines of its second odd-order mode, the resonant frequency of second odd-order mode is gradually moved down. As such, the first and second odd modes could be effectively moved in proximity to each other, thus resulting in the improved impedance bandwidth of the slot antenna under dual-mode operation. To validate the predicted results, the proposed wideband microstrip-fed slot antenna is in final fabricated and measured. Both simulated and measured results are derived with good agreement. Measured results indicate that the impedance bandwidth of the slot antenna has achieved an immense enhancement up to about 25.5% (2.43-3.14 GHz). Moreover, the front-to-back ratio (FBR) beyond 6.0 dB has been successfully obtained across the whole operating band for end-fire radiation.