In situ epitaxial growth of ordered FePt thin films with small and uniform grain size using RuAl underlayer is reported. A transmission electron microscopy image of a 20-nm RuAl layer deposited on a glass substrate revealed small (D~5.0 nm) and uniform (ΔDD~15%) grains. The (001) texture was formed in RuAl films at a substrate temperature higher than 100°C. The FePt L10 (001) texture with mean grain size of 6.63 nm and narrow size distribution (17%) has been successfully induced using a RuAl underlayer at a substrate temperature of 400°C. The influences of the RuAl composition ratio and Pt interlayer were studied.