Abstract: The connectivity between pores and fissures during in situ oil shale pyrolysis is an important element which controls shale oil and gas recoverable amount. However, conventional petrophysical testing methods can not cover all levels of pores and fissures in oil shales. Nuclear magnetic resonance (NMR) can show fluids in core pores and fissures, hence can be used to identify different levels of pores and fissures. We carried out NMR tests with oil shale samples by simulating the same conditions as 500 m underground and heating the samples to different temperatures. Results showed that the porosity of oil shales change according to temperature during in situ exploitation. Porosity increases from 250 to 350℃, decreases slightly from 350 to 400℃, and then increases again after 400℃. Permeability remains stable when temperature is lower than 400℃, increases by 10² times from 400 to 450℃, and increases by 10⁴ times at 500℃. The in situ retorting of oil shales should be made at a temperature higher than 400℃; however, oil shales underground might not reach 400℃ in many areas. In this case, we should explore at a higher temperature and heat for longer time, or fracture oil shales before heating.