Upper mantle velocity structure in the New Hebrides island arc region has been determined to a depth of 240km from the analysis of P and S wave travel times of 39 deep earthquakes using KAILA's (1969) analytical method. The present analysis reveals a linear increase of P wave velocity from 8.06km/sec at a 40-km depth to 8.19km/sec at a 240-km depth with a gradient of only 0.06±0.01km/sec per 100km. For S waves also, the velocity increases linearly from 4.55km/sec at a 40-km depth to 4.64km/sec at a depth of 220-km with a gradient of only 0.04±0.02km/sec per 100km. The velocity gradients for P and S waves, from 40 to 240km depth in the upper mantle beneath the New Hebrides arc are found to be extremely small as compared to those at similar depths in the adjacent Tonga-Kermadec-New Zealand region. Consequantly, the P and S velocities, to a depth of 240km., beneath the New Hebrides arc are found to be about 6% lower, on the average, than those in the Tonga-Kermadec-New Zealand region. This velocity difference is attributed primarily to very high upper mantle temperatures in the New Hebrides region which may be about 1, 000°C higher than those in the Tonga-Kermadec-New Zealand region. An alternative explanation based on existence of high temperatures in this region is also presented which explains deep seismic activity at 600km depth in the New Hebrides region without invoking the concept of a detached lithospheric slab proposed by some of the earlier workers.