N-doped activated carbons were obtained by using microalgae-sodium alginate (NaAlg) as renewable precursors due to the high nitrogen content of microalgae. One simple carbonization process with flowing N-2 gas only and no any other chemical agents or gases was used to prepare the activated carbons. Texture parameters, surface morphology and chemical properties of obtained activated carbons as well as the details of composition between microalgae and NaAlg have been investigated. By way of composition of microalgae and NaAlg, the tiny microalgae cells dispersed homogeneously inside the NaAlg and activated carbons with large specific surface areas over 1000m(2)/g were obtained. With high N content and appropriate pore size distributions, resulted activated carbons exhibited a very high CO2 adsorption capacity of 3.75 mmol/g, 1 bar, 25 degrees C and CH4 adsorption capacity of 7.27 mmol/g, 20 bar, 25 degrees C, which was much higher than commercial available coconut activated carbons (SBET 1350m(2)/g). The results suggested that microalgae-NaAlg composites derived N-doped activated carbons to be promising adsorbents for CO2 and CH4 and the composition method may provide us new idea for greener activated carbon preparation.