<p>An index value as an indicator of heartwood moisture content (MC) of sugi (<i>Cryptomeria Japonica</i>) green logs is proposed, based on longitudinal vibrational properties. In previous studies the average log MC was estimated from flexural vibration of logs using high negative correlation between specific Young’s modulus (<i>E</i>/<i>ρ</i>) and loss tangent (tan <i>δ</i>); however, it is difficult to estimate sapwood and heartwood MC separately using this method. Accordingly, we developed a quantitative index (<i>U</i>), in which the longitudinal vibrational properties of the log are substituted for the flexural vibrational properties. It has been reported that tan <i>δ</i> based on longitudinal vibration is larger than that from flexural vibration, given the large differences in the density of sapwood and heartwood. The index value <i>U</i> is smaller than the estimated average MC because of larger tan <i>δ</i> from longitudinal vibration when heartwood MC is smaller than average MC. The results of the vibration test (using 35 4-m sugi logs) showed that the index value <i>U</i> is well-correlated with heartwood MC. When six of the logs (in which the sapwood had begun to dry) were removed from the data analysis, the correlation coefficient was sufficiently high, at 0.78.</p>