To improve the CaO/CaCO3 thermochemical heat storage capacity and the mechanical property of CaO-based material during the repetitive heat storage cycles, CaO pellets were fabricated by the extrusion-spheronization method. The thermochemical heat storage performance of CaO pellets under pressurized carbonation (3-13 bar) and harsh calcination (950 degrees C, pure CO2) conditions were studied in a dual fixed-bed reactor. The effects of harsh calcination conditions, carbonation conditions, pellet particle size, and pore-forming agent were determined. CaO pellets carbonated under high pressure exhibit a better heat storage performance than calcined limestone. High carbonation pressure greatly offsets the negative effect of harsh calcination conditions on the heat storage capacity of CaO pellets. When the carbonation pressure is 13 bar, the carbonation conversion and heat storage density of the pellets calcined under harsh conditions reach 0.45 and 1423 kJ/kg after 10 cycles, respectively, which are very close to those calcined under mild conditions (750 degrees C, pure He). The improvement in heat storage capacity of the pellets becomes slight with increasing the carbonation pressure above 5 bar further. An appropriate carbonation temperature range is 800-850 degrees C, and a smaller particle size plays a positive but minor effect on the heat storage capacity of CaO pellets. The optimum heat storage conditions are obtained by the orthogonal experiments. The addition of biomass as a pore-forming agent in the pellets apparently increases the heat storage capacity of CaO pellets during the heat storage cycles. CaO pellets possess a high crushing strength of 4.7 N and high fragmentation resistance. CaO pellets prepared using the extrusion-spheronization method seem promising for CaO/CaCO3 heat storage.