In this paper the characteristics of flow boiling heat transfer in an annu-lus 2mm in width have been studied by using water and viscous liquid. The experimental results show that within a certain range of heat flux the flow boiling heat transfer coefficient in an annulus is larger than an empty tube by 80 %; the overall heat transfer coefficient is increased on an average by 60%. The calculated value of overall heat transfer coefficient is compared with the experimental value with water as the boiling liquid.