Microalgae gasification in the presence of Fe2O3 oxygen carrier and CaO additive was investigated in a fixed bed reactor to simulate the reactions in direct chemical looping gasification, focusing on the thermal behavior of CaO in synthesis gas production and multiple redox cycles. The results showed that Fe2O3 as oxygen carrier improved the synthesis gas but brought much more CO2 generation, lowering the gas LHV. Introducing CaO efficiently inhibited CO2 release and improved CO and H-2 production, and gasification efficiency and gas LHV elevated from 55.2% and 13.27 MJ/N m(3) to 66.2% and 14.85 MJ/N m(3) at 850 degrees C. CaO mainly played as CO2 absorbent and hardly had catalytic effects on tar crack for synthesis gas production at 650 degrees C. At 850 degrees C, CaO mainly played as catalyst and still had a great selectivity for synthesis gas production. The CaO/C molar ratio of 0.5 was sufficient for achieving its function. The multiple redox experiments showed that the gas efficiency and carbon conversion declined in the second reduction and kept at 60.1 +/- 1.2% and 77.4 +/- 1.5% respectively in 2-40 cycles. XRD and SEM-EDX results showed the coverage of CaO, the formation of calcium ferrites and CaO deactivation accounted for the variation of oxygen carrier reactivity in multiple cycles.