This paper presents the DC breakdown characteristics of a non-uniform electric field effected by the insertion of solid dielectric barrier under in air. The investigations have been done experimentally and simulated in order to study the breakdown phenomenon of solid dielectric barrier. The complicated breakdown voltage characteristics are studied experimentally under both positive and negative DC voltage polarity. The solid dielectric barriers are made of Polymethymethaacrylate (PMMA) with different thickness of 3, 5 and 8 mm, and the experiments are done under air pressure range between 0.5-3 bars. According to the experiments, the results are divided into three parts. The first part is the effect of pressure. The breakdown voltage increased when air pressure are increased. The second part is the simulation results of electric field distribution with solid dielectric barrier, which is numerically analyzed by Finite Element Method (FEM). It can be described that in case of the thicker solid barrier, the electric field stress at the contact point is lower than the one with thinner barrier. The third part is the effect of polarity on breakdown voltage. The results show the breakdown voltage increased when air pressure are extended and the intersection between the positive and negative DC breakdown curves at the pressure of about 1.5 bars. At the pressure lower than 1.5 bars, the negative DC breakdown voltage is lower than the positive case. In the other hand at the pressure higher than 1.5 bars, the negative DC breakdown voltage is higher than the positive case