Detailed heat flow measurements revealed an enormous heat flow variation (102 to 105 mW m-2) in the Jade hydrothermal field (27¿16'N, 127¿05'E and water depth 1350 m) located on the slope of the Izena Hole in the Mid-Okinawa Trough. Within the Jade site, heat flow is higher than 1000 mW m-2 and decreases to ~100 mW m-2 with a 1 km horizontal scale. Near the Jade black smoker, heat flow varies from >30,000 mW m-2 at a hydrothermally altered area to 1700 mW m-2 less than 100 m from it. A large-scale heat flow variation suggests that the base of the escarpment near the Jade site serves as a recharge area for the Jade site. Linear geotherms indicate that the upward Darcian flow within sediments is slower than ~3 cm yr-1. Thus the fluid circulation pathways would basically be restricted in permeable channels. Estimated conductive heat output rate from the Jade site is 4--7 MW, which is comparable to heat output by a single black smoker vent (~8 MW). One-dimensional thermal modeling predicts the existence of a boiling zone at ~200 m beneath the Jade site, which may be overlain by a fractured impermeable layer. The normal chloride content of venting fluids from the black smoker may be explained either by upwelling of fluid which boiled at shallow depth but suffered no phase segregation, or by upwelling of fluid above the boiling interface.¿ 1997 American Geophysical Union