  We report on a fundamental thickness limit of the itinerant ferromagnetic
oxide SrRuO$_3$ that might arise from the orbital-selective quantum confinement
effects. Experimentally, SrRuO$_3$ films remain metallic even for a thickness
of 2 unit cells (uc), but the Curie temperature, T$_C$, starts to decrease at 4
uc and becomes zero at 2 uc. Using the Stoner model, we attributed the T$_C$
decrease to a decrease in the density of states (N$_o$). Namely, in the thin
film geometry, the hybridized Ru-d$_yz,zx$ orbitals are terminated by top and
bottom interfaces, resulting in quantum confinement and reduction of N$_o$.
