To evaluate the Thomson effect on the temperature increase in Ge2Sb2Te5 (GST)-based phase-change random access memory (PCRAM), we created new dimensionless numbers based on Buckingham's Pi theorem. The influence of the Thomson effect on the temperature increase depends on the dominant factor of electrical resistance in a PCRAM cell. When the effect is dominated by the volumetric resistance of the phase-change material (C = rho(c)/Delta(x)/sigma << O(1)), the dimensionless evaluation number is B = mu(T)sigma Delta phi/k, where rho(c) is the contact resistance, Delta x is the thickness of PCM, sigma and k are the electrical k and thermal conductivities, mu(T) is the Thomson coefficient, and Delta phi is the voltage. When the contact resistance cannot be ignored, the evaluation number is B/(1 + C). The characteristics of hexagonal-type crystalline GST in a PCRAM cell were numerically investigated using the defined dimensionless parameters. Although the contact resistance of GST exceeded the volumetric resistance across the temperature range, the ratio of contact resistance to the whole resistance reduced with increasing temperature. Moreover, increasing the temperature of GST enhanced the influence of the Thomson effect on the temperature distribution. At high temperatures, the Thomson effect suppressed the temperature increase by approximately 10%-20%.