We report on thermal analysis of uncooled antenna-coupled La0.67Sr0.33MnO3 microbolometer made on circular SOI membrane with no limitation in its active area (circular membrane with diameter up to 2 500 μm). A simple method how to investigate the thermal conversion efficiency (thermal resistance value) is introduced. Thermal analysis is supported by the ANSYS modelling and simulation. It is found that Rth of our LSMO bolometer (bolometer sensitivity) can be tuned by the SOI membrane thickness. Rth of the bolometer as high as 164°C/mW is estimated from the thermal simulation for SOI membrane with the thickness of 300 nm.