An in-situ composite methodology was first proposed to prepare carbon supported PtNix (X = 1, 3, 1/3) via online reaction, nucleation and growth of nanocrystals (NCs) on carbon support along the microfluidic channel. Uniform dispersed ultra-small PtNiX NCs (1 nm(-3) nm) with high surface exposed Pt atoms and enhanced nanocrystal-carbon interface interaction can be constructed. The annealed nanocomposites with a Pt/Ni ratio of 1/3 exhibit a mass catalytic activity of 7.6 times higher than the commercial Pt/C nanocatalysts in the ethanol oxidation reaction. The chronoamperometry measurement indicates that their mass activity still retains 3.6 times of the commercial Pt/C nanocatalysts at 4000 s. Moreover, PtNi3/C nanocatalysts have the highest tolerance to CO poisoning according to CO stripping measurements. (C) 2018 Elsevier Ltd. All rights reserved.