Here, a support-type composite catalyst TiN/IrO2 with an outstanding catalytic activity for OER in acid electrolyte was prepared by a colloidal method. It was found the ultra-fine IrO2 nanoclusters (1.41 +/- 0.19 nm) scattered on the TiN support like strawberry seeds, which not only provided the higher active surface area, but also exposed much more surface unsaturated Ir atoms with the higher reactive activity compared to saturated iridium atoms. And the mesoporous structure and high surface area inherited from the TiN carrier were also maintained in the composite. Benefit from these characteristics, the as-prepared TiN/IrO2 with IrO2 loading of 31 wt% possessed a mass-normalized OER activity of 874.0 A g(-1)(IrO2) at the potential of 1.6 V that was about 5.0 times of the unsupported IrO2 (176.0 A g(-1)IrO(2)).