A thick film varistor comprising a thick film consisting essentially of 20 to 85 weight percent of tin oxide having an additive such as antimony oxide, and 15 to 80 weight percent of a glass frit such as a zinc barium borate glass. This thick film varistor is advantageous for its high n value (i.e. high voltage dependence of resistivity) and its low varistor voltage (i.e. voltage above which its resistivity abruptly decreases).