Preference value means livestock preference to eat some plants relative to other plants or species. Accordingly, by studying the preference values, the range managers could determine the rangelands grazing capacity to achieve the optimum performance of livestock and to guarantee the stability of rangeland ecosystem. The objective of this study was to determine the preference value of perennial grasses using two methods of preference index (species feeding frequency) and sheep grazing time (feeding time recording) in a semi-steppe rangeland in the middle Alborz Mountains of Iran during four consecutive years (2010-2013). The collected data were tested for normality and then, analyzed by the software of SAS9.1. The means comparisons were made using the Duncan method. Results showed that sheep preferred five grass species (Bromus tomentellus, Festuca ovina, Dactylis glomerata, Agropyron intermedium, and Agropyron trichophorum) in the rangeland. There were significant differences between species and years for grazing time method; however, there were no significant differences for preference index. Also, there were significant differences between months within years for both methods. Results showed that A. trichophorum with the average value of 13.8% grazing time was ranked as the first species followed by D. glomerata with the average value of 11.1% as the second one (Grazing time method). There were no significant differences between A. trichophorum, B. tomentellus, D. glomerata and F. ovina regarding the preference index with the average values of 0.57, 0.75, 0.70 and 0.56, respectively. Overall, the results indicated that grazing time percent method in semiarid rangelands gives more realistic results as compared to the preference index method because of the concrete and tangible results.