Actually many research are conducted on the ability of hydrogen-ammonia mixture fuel to replace classical fuel such as methane for gas (micro)turbine use. These studies concern the combustion stability and NOx formation. Temperature repartition in gas (micro)turbine combustor is very important from a thermomechanical point of view. This issue is often omitted in research. If modifications occur in temperature repartition, an overheating zone can appear and lead to mechanical damage of the combustor liner. Numerical studies, CFD method based with the use of Ansys Fluent tools, were conducted on a self-designed gas microturbine combustor, for methane application, using various ammonia-hydrogen mixtures and methane fuelling. Then, the obtained temperature maps were compared between methane and ammonia-hydrogen mixtures with various compositions to find an ammonia-hydrogen mixture composition that permits to reproduce a similar temperature repartition as for methane fuelling; this mixture contains maximally 10%mass hydrogen (or 48%vol). This ammonia-hydrogen mixture composition was compared to others research where the ammonia-hydrogen composition was optimised for combustion stability and NOx reduction (hydrogen content of 30%vol). According to performed studies, the proposed ammonia-hydrogen composition in others research is confirmed to be safe for gas microturbine applications from a thermomechanical point of view.