Abstract: Pretreatment of hempen pulp with sodium hydroxide were carried out, effects of NaOH concentration, pretreatment temperature and time on the yield, R-10 value, polymerization degree, iodine adsorption value and crystal form of fibers were investigated. The results showed that the dissolving ability of NaOH to the cellulose with low molecular weight was strong at low temperature . The lower temperature was, the wider of molecular weight distribution of cellulose dissolved was, and the cellulose Ⅰ transforms into cellulose Ⅱ more easily because of swelling by the NaOH solution. To increase NaOH concentration properly was helpful for the dissolution of hemicellulose and the low molecular weight cellulose in the pulp, and was useful to the sufficient swelling of cellulose. Cellulose Ⅰ was obviously transformed into cellulose Ⅱ as NaOH concentration was higher than 9 %, and resulted in decreasing of the accessibility. Effects of pretreatment time between 1 and 3 hour on the yield of fibers, R10, polymerization degree, adsorption capacity for I2 and cellulose crystal were not obvious contrast to temperature and concentration.