AbstractPulp fibers were treated in aqueous NaOH/urea/thiourea solution at low temperatures (from -14 °C to 8 °C) to prepare high-permeability filter papers. The effects of treatment temperature and time were investigated to control the permeability of the filter paper. SEM images were taken to observe the physical configuration of fibers, and fiber quality analysis was used to characterize the properties of the fibers. The main parameters of the filter papers (permeability and bulk) were increased markedly. The permeability was increased from 150.1 L/(m2·s) for the untreated paper to 1136 L/(m2·s) for the treated paper, and from 4.3 cm3/g for the untreated paper to over 5.5 cm3/g for the treated paper. The zero-span tensile strength changed only slightly. Moreover, the characteristics of the pulp fibers underwent some positive changes. These results demonstrate that the permeability of paper sheets can be preferably improved by treating fibers in NaOH/urea/thiourea solution at -2 °C for 30 min.