A visualization system was designed for a real-time observation of scales during direct contact membrane distillation (DCMD). The system was composed of a MD module, a camera and light sources. Firstly, visibility of the scales with the system and correlation between detected spots and wetting were assessed during the MD process using a polyvinylidene fluoride (PVDF) 0.45 mu m membrane. The white spots were observed in the captured images from the visualization system and got broader with the electrical conductivity (EC) increase in permeate. It was verified that the spots indicated the pores blocked by the CaSO4 scales. The wetting occurred through the white spots if the pressure was applied. Therefore, the high wetting potential area could be identified with the suggested visualization system during the MD operation in real-time. The visualization system was applied to the MD process under different permeate temperatures (20 and 55 degrees C). The visualization system successfully detected the pore blocking related with different scale formation according to the temperatures at the membrane surface.