The ab initio full-potential linearized augmented plane-wave (FP-LAPW) method was employed to investigate the magnetic properties of 1 monolayer (ML) Fe on W(100) and W(110) substrates. Theoretical study reveals very different characteristics arising from the Fe-Fe and Fe-W interactions in these two systems, which include Fe-W relaxations, magnetic moments, and interlayer exchange couplings. The origin of these differences is considered by the crystal-field effects in these two crystallographic orientations. We show that symmetry plays a crucial role in the Fe-Fe and Fe-W interlayer interactions in Fe/W ultrathin magnetic films.