A linear resistive tearing instability of a thin magnetic reversal layer, with thickness down to the order of the ion inertial length, is analyzed under an incompressible approximation. The growth rate is calculated numerically for an oblique tearing mode exp[ik(zcos&thgr;+ysin&thgr;)>, where &thgr; is the propagation angle between the wave vector k and the z axis parallel to the reveral magnetic fields. In the MHD limit, it is known that the maximally growing mode is centered about &thgr;=0, and the growth rate decreases monotonically with increasing ‖&thgr;‖. When the finite-ion-inertial-length effect is included, the tearing instability is either enhanced or suppressed depending on the propagation angle &thgr;. We find that only waves in several relatively narrow bands of the angle &thgr; are unstable and that the unstable tearing modes become highly drifting for oblique propagations. Either the ion-inertial-length effect or the oblique propagation can lead to a vortex electric current which is responsible for magnetic field By components normal to the tearing plane. The structure of magnetic islands is fully dimensional and is carefully examined. Implications of our results for the patchy and intermittent reconnection of a finite-size current sheet at the dayside magnetopause are discussed. ¿American Geophysical Union 1991