Co-combustion with coal in coal-fired boilers has been developed rapidly as an effective treatment method for sewage sludge. Sometimes wet sludge is directly used for co-combustion. Harmful gas emission from the co-combustion was investigated in an isothermal tube furnace. Results showed that the CO, SO2, and NOx emissions from the co-combustion had no large increase compared with coal mono-combustion when the blending ratio of wet sludge was 10-50%. When the blending ratio of wet sludge was 30%, the gaseous emission was lowest and the conversion rates of S to SO2 and N to NOx were 1.6 and 1.1%, respectively.