Abstract The torrefaction performances of microalga Nannochloropsis Oceanica under oxidative and inert atmospheres are characterized and compared with each other based on several operating parameters. By conducting several comparative indexes, the results suggest that oxidative torrefaction is more capable of upgrading microalgae due to its relatively lower solid yield and energy input, as well as relatively higher enhancement factor and upgrading energy index. Compared to inert torrefaction, the indexes indicate that oxidative torrefaction at 250 °C for 30 min has higher energy yield (1.02 times) and energy efficiency (2.2 times) but whereas lower energy input (0.4 times). With increasing torrefaction severity, the pyrolysis curve gradually becomes smooth and shift to a high-temperature zone. The peak temperatures of torrefied microalgae present an increasing trend, especially in the oxidative atmosphere. After oxidative torrefaction, microalgal solid biofuel is upgraded as peat and lignite, from the viewpoint of elemental composition. Furthermore, oxidative torrefaction is more suitable than inert torrefaction for producing bio-oil which mainly contains dianhydromannitol, neophytadiene, and palmitoleic acid. The TG-FTIR-MS results uncover the pyrolysis characteristics and reactivity of torrefied microalgae, and elucidate that oxidative torrefied microalgae is more reactive.