A numerical method was developed to calculate the flow field and filtration drag around nanofiber/microfiber with consideration of gas slip effect, and the effects of Knudsen number Knf and the fiber volume fraction C on velocity distribution around the fiber surface and fiber filtration resistance were discussed and analyzed. The results indicated that significant differences could be found between flow field around the fiber in slip flow regime and in no-slip flow regime for nanofiber/sub- microfiber filtration, especially for the case of high fiber volume fraction where the maximum slip velocity along the fiber surface approached the filtration velocity, however, the gas slip effect had a negligible influence on flow field and filtration resistance for large-scale microfiber filtration. Moreover, existing filtration resistance coefficient models developed for fibrous filter were inadequate in predicting nanofiber filtration resistance coefficient in slip flow regime, and orders of magnitude deviation could be seen between the different filtration resistance coefficient expressions. Based on the simulation results, a reliable expression for the nanofiber/microfiber filtration resistance coefficient in the range of 0.01≤C≤0.1 and 0.013≤Knf≤2 was developed.