This study compared the sole pyrolysis of sewage sludge (SS) or rice straw (RS) with co-pyrolysis of SS /RS at different mass ratios (1∶ 3,1∶ 1,and 3∶ 1) regarding with biochar production,C and N transformations and elemental composition. The pyrolysis were carried out with a tube fixed-bed reactor at different temperatures of 300 ~ 700 ℃. The results indicated that co-pyrolysis of SS and RS was not a simple addition of the individual contribution of the two. Co-pyrolysis of SS and RS had no synergistic effect on the biochar yield. However,the synergistic effect of co-pyrolysis on fixedcarbon yield was dependent on co-pyrolysis conditions. At the same pyrolysis temperature,the values of synergistic effect (Deltas) of carbon and nitrogen contents in biochars from 1∶ 3 and 3∶ 1 SS /RS were significantly higher than those from 1∶ 1 SS /RS. The maximum DeltaC of 12. 43% and the maximum DeltaN of 40. 65% were observed using a mass ratio of 1 ∶ 3 SS /RS and 3 ∶ 1 SS /RS at a pyrolysis temperature of 400 ℃,respectively. As the pyrolysis temperature increased,the synergistic effects of co-pyrolysis on C and N fixing increased,reaching a maximum value of 53. 77% ~ 56. 13% and 38. 30%~ 39. 12%,respectively. This suggested that co-pyrolysis is benefit for fixing C and N in biochar. Except that the atomic ratio of H/C in the biochar from 3∶ 1 SS /RS was higher that its theoretical value,the atomic ratios of H/C,(O + N) /C and O/C were all lower than their theoretical values under other co-pyrolysis conditions (i. e. ,SS /RS ratio,temperature) . This indicated that co-pyrolysis had evident positive synergistic effects on biochar stability. The synergistic effects of co-pyrolysis of SS and RS were enhanced by deoxygenation and dehydrogenation,dehydrogenation,and deoxygenation when SS /RS was in ratios of 1∶ 3,1∶ 1 and 3∶ 1,respectively. The results showed that the effect of carbon and nitrogen fixation in biochar can be significantly enhanced using co-pyrolysis of SS and RS with an optimal mixture ratio,and this provides a new technology for C and N sequestration to mitigate climate change.