A powder coating composition is disclosed which includes (a) 100 parts by weight of a mixed epoxy resin having an epoxy equivalent of 800-2000 and containing (a-1) a diglycidyl ether of bisphenol A having an epoxy equivalent of 180-2500 and (a-2) a rubber-modified epoxy resin having an epoxy equivalent of 180-2500 and obtained by reaction of a diglycidyl ether of bisphenol A with a carboxyl terminated acrylonitrile-butadiene copolymer; (b) a mixed curing agent consisting of (b-1) 0.05-5 parts by weight of an imidazole compound, (b-2) 0.5-10 parts by weight of dicyanodiamide and (b-3) 1-10 parts by weight of a dihydrazide of a dibasic carboxylic acid; and (c) 50-100 parts by weight of silica.