The microscopic pore structure of tight oil reservoir rocks in Lucaogou Formation was investigated using high pressure mercury intrusion（HPMI），rate-controlled mercury intrusion（RCMI），and scanning electron microscopy（SEM）.RCMI can quantitatively distinguish the pore body and pore throat of the sample，and can accurately determine the pore size of the large pores. However，because of its lower mercury intrusion pressure，the RCMI cannot detect the small pores in the samples. HPMI with a higher mercury intrusion pressure can accurately determine the pore size of small pores of samples. The connected points of two techniques were obtained using interpolation method in the common measurable interval，and then the combination of HPMI and RCMI was used to obtain the overall pore size distribution of the tight oil reservoir rocks of Lucaogou Formation. The results demonstrate that the types of pores of the tight oil reservoirs in Lucaogou Formation are mainly intergranular pore，residual interparticle pores，and intercrystalline micropores. All the capillary pressure curves of these rock samples have no intermediate gentle section. The pore structure of these samples is complex，and the sorting is poor. The pore size distribution pattern of the tight oil reservoirs of Lucaogou Formation in Jimusaer Sag are bimodal. According to the pore size classification scheme proposed by LOUCKS，the nanopores with radii ranging between 0.07 and 1 μm are abundant. The radii of mesopores are around 120-150 μm，and the micropores are rare compared to nanopores and mesopores. Because HPMI and RCMI are based on the same physical procedure-mercury intrusion，the combination of these two techniques is an effective method to accurately obtain the overall pore size distribution of the tight oil reservoir rocks.