The effect of plasticizer on dielectric properties of poly(methyl methacrylate) (PMMA)/titanium dioxide (TiO2) composites was investigated. Propylene carbonate (PC) was used as plasticizer in the samples which were prepared with the conventional solvent casting technique. Scanning Electron Microscopy with Energy Dispersive X-Ray Analysis (SEM-EDX) and Differential scanning calorimetry (DSC) analyses and LCR Meter measurements (performed between 300 K and 400 K), were conducted to examine the properties of the composites. With the addition of plasticizer, the thermal properties have changed and the dielectric constant of the composite has increased significantly. The glass transition temperature of pure PMMA measured 121.7 degrees C and this value did not change significantly with the addition of TiO2, however, 112 degrees C was measured in the sample with the addition 4 ml of PC. While the dielectric constant of pure PMMA was 3.64, the epsilon ' value increased to 5.66 with the addition of TiO2 and reached 12.6 with the addition of 4 ml PC. These changes have been attributed to increase in amorphous ratio that facilitates polymer dipolar and segmental mobility.