Tube cyclic expansion-extrusion (TCEE) is proposed as a novel severe plastic deformation method for processing cylindrical tubes. TCEE is capable of imposing large strains to cylindrical tubes without dimensional and geometrical changes. In TCEE process, peripheral small shallow grooves were devised on mandrel and inner surface of the chamber. The grooves allow tubular specimen to initially expand and then extrude to the initial thickness while it is deformed between mandrel and chamber. TCEE was first trialed on commercial AZ91 magnesium alloy and substantial grain refinement was recorded. Mean grain size of 1 mu m was achieved after two cycles of TCEE. Microhardness assessment across tube thickness demonstrates good homogeneity of hardness distribution; microhardness of the initial tube increased from 65 to 90 Hv after two processing cycles of TCEE. After two processing cycles of TCEE, yield and ultimate strengths were increased by 2.9 and 2.6 times compared to those in as-cast condition. The elongation to failure was also increased from initial value of 1.6 % to about 8.1 %. The process was also numerically simulated by commercial FE code of ABAQUS/explicit to further investigate strain accumulation in TCEE process.