Thermal drying behavior of the municipal sewage sludge in nitrogen atmosphere was explored using a thermal analysis technique under isothermal and nonisothermal drying conditions. The Midilli model, $$ {\text{MR}} = \exp ( - kt^{\text{n}} ) + bt $$ MR = exp ( - k t n ) + b t , was the best suitable for predicting both the isothermal and nonisothermal drying behavior of the sewage sludge with the highest R 2. The isothermal drying apparent activation energies of the first falling rate period and the second falling rate period were 18.03 and 11.87 kJ mol−1, respectively. The nonisothermal drying apparent activation energies of sewage sludge were from 33.61 to 47.37 kJ mol−1 in the first falling rate period and from 20.47 to 33.43 kJ mol−1 in the second falling rate period, respectively. In two falling rate periods, the dominant mechanism functions for the isothermal drying were identical, $$ - \ln (1 - \alpha ) $$ - ln ( 1 - α ) . The dominant mechanism functions for the first falling rate period and the second falling rate period in the nonisothermal drying were described by $$ [ - \ln (1 - \alpha )]^{1/2} $$ [ - ln ( 1 - α ) ] 1 / 2 and $$ [ - \ln (1 - \alpha )]^{1/3} $$ [ - ln ( 1 - α ) ] 1 / 3 , respectively.