Dissolved Poly (methyl methacrylate) (PMMA) in Dimethylformamide solution was fabricated as thin films on different substrates, glass, poly (ethylene naphthalate) and Si wafer, using spin coating technique. The effect of different substrates on electrical performances of the PMMA films was investigated. For an acceptable organic thin-film transistor (OTFT) performance, uniform thickness with low defect density of the PMMA thin-films was required. A parallel plate capacitor structure was used to test electrical properties of the PMMA dielectric layers. Good dielectric properties were obtained on glass and PEN at film thicknesses of 95 nm. An optimal condition for the OTFT preparation was used to fabricate an OTFT with the PMMA dielectric layer on glass substrate. Measured results showed that the OTFT achieved a mobility of 0.16 cm2/V.s, a threshold voltage of -3.6 V, and on/off current ratio of 1×105.