Fluorite-structured yttria-bismuth oxide membranes with improved chemical stability were prepared by doping samarium in the lattice. The samarium-doped yttria-bismuth oxide (BYS) membranes are in fluorite structure, with high oxygen permeability and good catalytic properties for oxidative coupling of methane (OCM). The oxygen partial pressure dependence of oxygen permeation flux shows a shift of the rate-limiting step from the surface reaction to the bulk electron-hole conduction as temperature increases from 750 to 950 degreesC. OCM reactions were conducted in a membrane reactor made of the disk-shaped BYS membrane. Methane- and oxygen-containing streams are fed into the opposite sides of the BYS membrane to allow OCM reactions to occur on the membrane surface exposed to methane. The membrane reactor gives C-2 yields of 4-11% with C-2 selectivity up to 74%. The performance of this membrane reactor is closely related to oxygen permeation properties of the BYS membrane. Experimental results show the effectiveness in improving the C-2 yield by increasing the membrane surface area ro reactor volume ratio. In this disk-shaped membrane reactor the highest C-2 yield achieved is 17%, with a C-2 selectivity of about 80%.