The influences of nitrogen addition and the attenuation of microwave power during microwave plasma-assisted chemical vapor deposition (MPCVD) of diamond films on electrochemical properties of the grown films were investigated. Although diamond films grown in the presence of a small amount of nitrogen did not contain sp(2) carbon and appeared to be normally deposited films, the electrical and electrochemical properties were different from films grown under an atmosphere without nitrogen. Especially, the potential windows of diamond films grown in the presence of nitrogen at low microwave power were very wide and extended to 3.6 V in sulfuric acid solution, which is comparable to the widest value of the potential window of diamond electrodes reported till now. It was also found that the attenuation of the plasma power during MPCVD by itself was effective in the expansion of the potential window because of the defect-free growth of diamond crystals.