Electrical transformers are considered one of the very important elements on the operation of power systems. The operation of electrical transformers is dependent on the performance of transformer oils. Hence, it is very necessary for continuous and on line checking the transformer oil insulation level. Transformer oil wet process generates oil discharge space charge in the long run and bubbles are formed. When the electric field reaches a certain limit, partial discharge appears and lead to deterioration of oil insulation level. Multi-wall carbon nanotube (MWNT) has many advantages for detecting PD of oils (e.g., high surface to volume ratio). So, we reported the performance of (MWNTs) films sensor for continuous and on line monitoring the oil insulation level. MWNTs sensors were synthesized using chemical treatment. Treatments of MWNTs sensors were made by using different acids. Specifications of nanotubes were made using microscope (operate in two and tree directions) and fourier microscope. Experimental results show the performance of nanotube sensors is very good.