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This paper presents a method for estimating the absolute lock effect in bolted joints during off-road vehicle operation. There is a good linear relation between axial tension decrease tendency (Loosening phenomenon) and the operation time (or mileage or number of operations) after the tightening on logarithmic coordinates. Based on this relation, this report leads to two estimating methods described below. 1) Decrease of axial tension (self-loosening) is estimated accurately after long hours since tightening by measuring the initial axial tension behavior using the bolt loosening evaluation diagram. 2) Method of Estimating the locking device availability (usefulness) on actual machine operation is obtained from laboratory loosening test results.