Abstract: The objective of this work was to study the effect of paper mill waste material and sewage sludge mixture on acid soil properties. For this purpose, the acid soil was amended with a mixture of sewage sludge and paper mill waste material (1:1 by weight) in two different proportions (2% and 4%). An incubation experiment (60 days) was conducted to study the effect on different chemical, physical, and biological properties of soil. Results showed that final acid soil pH increased from 5.2 to values greater than 6.4, total organic carbon content increased by greater than 15%, and the stability of soil aggregates significantly increased after soil treatment. Also, soil respiration increased between 452% and 962% according to the values of the total mineralization coefficient. Last, thermal analysis of samples of amended soil showed an increase in the thermostability index, which is a reliable parameter for evaluating the degree of maturation of soil organic matter after organic amendment addition.