In this study thermoecological cost (TEC) analysis of hydrothermal carbonization (HTC) of under-sieve fraction (USF) of municipal solid wastes (MSW) was performed. The TEC of hydrothermal carbonization of the USF was studied, considering two DB/W conditions (0,07 and 0,15), three different operative temperatures (180-200-220°C), and three different operative times (1-4-8 h). The influence of the single processes and operative conditions were analyzed and compared assuming different USF treatment processes (bio stabilization and landfilling, HTC and wastewater treatment (WWT). Thermo-ecological cost, which in contrast to other methods of ecological assessment, can bring all environmental impacts into one measure which is the exergy of the consumed natural, non-renewable resources is resulted in an appropriate method for evaluating the environmental performance of fuel production processes. The best TEC balance was obtained for the case of HTC where the substrate is treated for 1h under 180°C temperature conditions and with a dilution of 0.15. It is demonstrated that the USF can be effectively transformed into hydrochar. Even if, the base treatment of USF is a simpler process, HTC process results are more favorable with respect to the base case.