The polymeric melt viscosity depends on shear rate, temperature and pressure. But the effect of pressure is often neglected. In this work, the pressure dependency on shear viscosities had been investigated on a commercial high pressure slit-die rheometer with back pressure device. By assembling the micro-rheology slit-die in series with a standard slit-die, we are able to increase the pressure on the polymer melt in the slit. The pressure levels are varied by changing the available geometrical dimension of the micro-rheology slit-die. One semi crystalline polymer (PP) and one amorphous polymer (PMMA) had been analysed. Results show that the effect of pressure on viscosity is significant. The amorphous PMMA shows a higher pressure dependency than the semi crystalline PP. In addition to pressure effect on viscosity, the pressure effect on polymer melt temperature during compression was also investigated. The compression heat tests were carried out with the help of thermocouple integrated counterpart of test cylinder designed by Institute of Plastics Processing at the University of Leoben. Results show that the melt temperature increases linearly with pressure rise. For PP increase in melt temperature is about 5.3°C at 1000 bars. Results of compression heat help to improve the accuracy of the calculated pressure coefficient value.