Provided is a process for producing electrode catalyst fine particles capable of affording formed catalysts such as membranes excellent in uniformity and strength and also employable as catalyst to exhibit high catalytic activity over long periods because fine metal particles are highly distributed on carriers to make particle growth or the like less likely to occur. Electrode catalyst fine particles comprise fine particles which have an average particle diameter of 10 nm to 10 .mu.m and in which fine metal particles are supported on carrier particles, wherein a potential difference (absolute value of (II) minus (I)) between a flow potential (I) of a water dispersion of the fine metal particles at 1 wt % concentration and a pH of 3 to 8 and a flow potential (II) of a water dispersion of the carrier particles at 1 wt % concentration and a pH of 4 to 10, is in the range of 10 to 3000 mV.