Advice from forest machines performance under different site conditions can be the base of accurate scheduling and increasing of machine usage. Skidder Timberjack 450c is most common machines in northern forests of Iran. Three different forest sites in low height, middle height and high height were selected to compare the hourly production performance of this skidder. The elements of skidding cycle was determined and then 48, 40 and 55 skidding cycle was studied using continues time study method. After data analysis the mathematical model was obtained to predict the skidding time in each site. This model in low height was the function of skidding distance, route slope, volume and number of timber. In middle height was the function of skidding distance, volume and number of timber and in high height was the function of skidding distance, route slope, winching length and slope. The mean number of timber decreased from low height to high height in each cycle and the mean volume of timber increased. The hourly production (with calculation of pure time) was 6.57 m3/h in low height, 8.42 m3/h in middle height and 9.57 m3/h in high height. The hourly production of this skidder in different sites with different elevation at sea level was significantly different