A resin composition superior in stability and external appearance characteristics during/following melt molding; and an extrusion-molded article, an injection-molded article and a blow-molded article that are formed from the resin composition are provided. The resin composition contains an ethylene-vinyl alcohol copolymer as a principle component, and has a heterogeneous nucleation index (f) of less than 0.6 as determined by formula (1) based on a differential scanning calorimetry (DSC) curve obtained by DSC in which the resin composition is cooled at a rate of 150 °C/sec from a molten state at 210 °C. In the formula (1), Q total represents an area of a total region surrounded by the DSC curve and a base line, and Q hetero represents an area of a heterogeneous region that is a part of the total region, falling within a range from the temperature lower than the melting point by 38 °C to a temperature lower than the melting point by 75 °C. f = Q hetero / Q total