A marked feature of the pebble divertor system is an effect caused by the use of a functional multilayer coated pebble, which consists of a surface plasma facing layer, an intermediate tritium permeation barrier layerand a kernel for heat removal. The dimensions, structure and irradiation conditions of the pebbles are the important issues for the development of the pebble divertor. From the viewpoint of resistance to the induced thermal stress,the pebbles aretaken to be as small as possible. On the other hand, from the viewpoint of pumping performance, the suitable irradiation temperature range of the surface layer of the pebbles was estimated from experimental results and anumerical analysis. A pumping process enhanced by dynamic retention is available to extend the upper limit of the allowable irradiation temperature range from 900 to 1100 K. On taking the temperature rise limitation due to thepumping effect and the fracture strength due to the induced thermal stress limitation, it was found that a pebblediameter of 1-2 mm is possible in about 20 MW/m2 for the SiC kernel and one of2-3 mm in less than 30 MW/m2 for the graphite kernel.