Viscosities and thermal properties of glasses in the system PbO-SiO2 were measured at a temperature region of glass transformation. The glass transition temperature values were decreased with an increase in the PbO content. The measures of fragility $Delta$Cp, and Eη/Tg where $Delta$Cp, Eη and Tg are respectively the heat capacity change in the glass transition the activation energy for viscous flow and the glass transition temperature were increased monotonously in the glasses with low PbO content(<50 mol%) The measures of fragility $Delta$Cp, and Eη/Tg did not change in the glasses with high PbO content(>50 mol%) As a results of these measure-ments a good correlation was observed between $Delta$Cp, and Eη/Tg in the system PbO-SiO2