Nickel particles similar to 300 nm in diameter were fabricated by electroless deposition using hydrazine as a reducing agent in nickel hydroxide/ethylene glycol suspension at 353 K without any dispersing agent. The formation mechanism of nickel nanoparticles is discussed from the viewpoint of thermodynamics with in situ monitoring of nickel deposition and mixed potential. Specifically, in situ monitoring of mixed potential in combination with thermodynamic calculation is useful in discriminating whether or not nickel will be deposited in a reaction. The mixed potential drastically changed at the end point of the nickel deposition reaction, indicating that the cathodic reaction, which determined the mixed potential, switched from the nickel deposition reaction to hydrogen generation reaction. (C) 2008 The Electrochemical Society.