The results of studies on early competences suggest that the child's delay of gratification (DoG) ability is a major predictor of school readiness (SR). However, there are no direct data or convincing explanations as to how DoG translates into preschool children's readiness to commence school education. Aiming at a better understanding of the relationship between DoG and SR, standard solutions to measure these variables have been employed, i.e. the marshmallow task proposed by Mischel, and the Intelligence and Development Scales for Children (IDS). The study was conducted on a sample of 68 six-year-old children. In addition, the effects of selected socio-demographic variables were controlled. Contrary to what was expected, the delay of gratification proved to be a poor predictor of school readiness among six-year-old children. The study results indicate that DoG acts as a mediator in the correlations between school readiness and children's age and place of residence. The future studies should be considered in the context of longitudinally measurement and some changes in marshmallow task, for example, more attractive stimuli.