The effective thermal conductivity of the particle layer between heat exchanger tubes and their wastage covers was measured by using a 2 MWth pressurized fluidized bed test rig to design in-bed heat exchanger tubes with wastage covers in pressurized fluidized bed combustors. Heat exchanger tubes with wastage covers having two different gap lengths and without wastage covers were prepared. An increase of the gap length increased not only the loading rate of in-gap particles but also the effective thermal conductivity of the particle layer. The effective thermal conductivity of the particle layer over a wide range of particle temperature was investigated by use of a laboratory scale effective thermal conductivity test rig and an atmospheric pressure bubbling fluidized bed combustor. It was found that the effects of gap length on the effective thermal conductivity of the particle layer can be predicted by a modified Kunii-Smith's empirical equation for estimation of the effective thermal conductivity in a packed bed. The shape factor in the equation, beta, is taken as 3.4 for estimation of the effective thermal conductivity of the in-gap particle layer in pressurized fluidized bed combustors.