As a new kind of advanced cooling technique, steam cooling has been applied in modern high temperature gas turbine blade cooling for improving the turbine efficiency. The superheated steam is selected as coolant to replace traditional compressor air as closed loop steam cooling for the internal convective cooling. This paper experimentally and computationally investigates the cooling performance of internal steam convective cooling in a nozzle guide vane with five smooth radial cooling ducts. Experiments are conducted on a linear turbine cascade at exit Mach numbers of 0.9, and exit Reynolds number of 1.2 x 10(6). Temperature and static pressure on the cooled vane surface are measured at the mid span for a range of coolant-to-mainstream temperatures ratio and coolant-to-mainstream mass flow ratio. The numerical investigations using the conjugate calculation technique are also performed to predict the complex three dimensional flow and heat transfer. The k-omega based Shear-Stress-Transport (SST) model is selected as the turbulence model. It can be found that the numerical results of vane temperature are underestimated compared with experimental data, especially at the trailing edge. The coolant steam has much higher cooling effectiveness than air, about 12%. The cooling effectiveness at the vane middle chord region is much higher than that at the leading and trailing region, by approximately 50% and 20%, respectively, which will lead to great temperature gradient and thermal stresses at the leading and trailing region. Therefore, more complicated cooling configuration besides convective cooling may be necessitated for this vane. (C) 2013 Elsevier Ltd. All rights reserved.