The influence of milk dry matter (DM) content (8.20%, 9.27% and 10.28%) and applied heat treatments (untreated milk and milk heat-treated at 90°C/10’) on the viscosity of set-style yogurt and acid casein gel gained by acidification with GDL (glucono-d-lactone) has been investigated. Viscosity was measured during the time of 3 minutes at constant speed of spindle rotation of 20 rpm. The results have shown that yogurt samples produced from untreated milk had higher viscosity values than samples produced from heat-treated milk. An increase of dry matter content influenced the increase of viscosity of yogurt samples produced from both untreated milk and heat-treated milk. Samples with 10.28% DM had the highest viscosity values. An increase of DM content in samples of acid casein gel produced by acidification with GDL had an opposite influence. The samples with smallest DM content had the highest viscosity values. There is no linear relationship between the period of storage of set-style yogurt produced from heat-treated milk and viscosity value.