This issue in hand is the result of terrain classification according to accessibility, to have a functional one for using of harvesting machinery in northern forest of Iran. New concepts of soil stability and the relationship between soil strength and accessibility, have been found. The results of functional classification for the use of harvesting machinery in relation with slope percent are as following: 1. According to slope percent, Parcelle No. 11 has the easiest terrain condition for ground skidding. 2. We are able to select standard tractors for wood transportation in more than 50% of passable area. 3. 49% of study area are passable through ground skidding machines. To determine the rate of soil strenghtʾs effect, we made an enquiry on soil condition has been made. Soil is the best representative factor which demonstrates the capacity of ground stability to support vehicular off-road movement and interaction between vehicle and soil. Soil strength was specified by determinating soil texture and drainage. Then microtopography of study area has been studied, since lack of aerial photography with scale 1:5000 and inharmonious distribution of ground roughness an integrated inventory have been performed. A descriptive classification with 3 parameters has been made to provide a functional classification, three maps extracted from slope, soil strength and microtopography tables were overlaid. The new one is called utilization unit map. This new map and functional classification for use of harvesting machinery were demonstrated as confirmation and restriction of using ground skidding methods, also type of machines in study area which using them have least erosion hazards. The climate condition has also a great effect on wood transportation quantities which shows differences in soil stability between two different moisture condition (saturated and optimum condition). The fluctuation of soilsʾ moisture in different seasons have the same situation with above. This research as a classification system was the first enquiry which have been tested in practice in northern forest of Iran. The results are as follows: 1. Determination of slope percent is not enough to predict accessibility of forest by harvesting machines. 2. Climate conditions which were introduce as an optional parameters in fifteen session of FAO held at IZMIR (Turkey) in 1984 is suggested as an obligative factor in northern forests of Iran. 3. Soil strength is the other obligative parameter which was restricted ground skidding in forests (in the study area 15% of passable area were restricted by this parameter). 4. Off-roud movement construction has a great effect on selecting suitable type of machines and accessibility was raised from 10% up to 37% of area by constructing 3 new skidding roads. 5. Restrictive and obligative oarameter in this research were: soil strength, slope, climate condition and density of skidding road. Ground roughness (microtopograpy) had not a great value. It is needed to do more enquires on it. In this research that was only an optional parameter which has no significant effect on accessibility, also we need more enquires on siliviculture and forest management subject to determine exactly the quantity of each machines and reduce different aspects of erosion hazards.