The performance of N type organic thin film transistors in the atmospheric environment are unstable. In order to improve the stability of transistors in the atmospheric environment, we fabricated a single insulating layer of SiO2 device and SiO2/PMMA double insulating layer devices, using novel N type material PTCDI C8 as the active layer, and Ag as source and drain electrodes. Both of them were packaged by poly p xylylene for measurement of I V curve. The morphology and crystal structure of the active layer have been analyzed. The results reveal that the devices with PMMA as insulating layer has a good performance, the field effect mobility and switching current are better than that of devices using SiO2 as insulation layer. It can be concluded that the materials and technology of packaging in our experiments can maintain the N type organic thin film transistor performance.