This article presents a thermal biosensor to diagnose anemia without chemical treatments using temperature increases of red blood cells when hemoglobin molecules absorb photons of specific wavelengths and convert them to thermal energy. For measuring the temperature changes, a micro-scaled platinum resistance temperature detector was developed. We also designed and fabricated a thermostat system to maintain a constant ambient temperature (<±0.1°C). An 8.0W/cm2 diode pumped solid state (DPSS) continuous wave (CW) laser module with a wavelength of 532 nm was used to heat the red blood cells. Using this system, we successfully measured temperature variations of whole blood samples from 10 anemic patients and subsequently determined the concentration of hemoglobin. The method proposed in this paper requires significantly smaller quantities of whole blood (6μl), as compared with conventional methods (175μl), and allows instantaneous diagnosis (6 seconds) of anemia.