Traction performance of off-road vehicles is estimated using slip-traction relationships Two zero condition accepted by ASAE have been used widely to obtain the slip-traction relationships of off-road vehicles. This study was carried out using the soil bin systems to investigate the characteristic of slip-traction curves obtained using two zero conditions defined by ASAE. which are driving and driven zero condition, and to present disadvantage of slip-traction relationship based on two zero conditions of ASAE. The results of this study are summarized as follows : 1. For the driving zero condition, the curve of slip-traction relationship shows some issues. The first question is that the slip is zero when the traction is zero. The second question is that the value of slip is smaller than that of corresponding real slip, as the rolling radius decreased f3r the setting zero condition with driving wheel. 2. For the driven zero condition. slip occurs when the traction is zero, which is more realistic results than driving zero condition. But when a zero condition is set, skid occurs and this result increased the rolling radius of tire and increased slip value f3r the specific traction value of whole slip range. This kind of trend was getting bigger as the soil is softer, or the tire inflation pressure is higher. 3. From the results of this study, it was found that slip-traction relationship obtained by two zero conditions of ASAE is not realistic in estimating the traction performance of off-road vehicles. And also slip-traction relationship obtained for the same experimental condition showed different result in accordance with chosen zero condition,