Structure-controlled Pt-based catalysts have been known to exhibit improved electrocatalytic activities due to particularly modulated surface structures favorable for alcohol electrooxidation. We report PtNi alloy dendrite structure for formic acid electrooxidation prepared by a thermal decomposition method. The Pt-Ni nanodendrites display 3-dimentional nanostructure and homogeneous alloy formation between Pt and Ni. In formic acid electrooxidation, the Pt-Ni alloy nanodendrite catalyst exhibits much improved electrocatalytic activity.