Human HtrA2 belongs to a new class of oligomeric serine protease, members of which are found in most organisms. Mature HtrA2 is released from mitochondria into the cytosol in response to apoptotic stimuli. In this report, the effect of temperature on proteolytic activity of HtrA2 and related structural properties were investigated. In the range from 25 to 55 degrees C, the proteolytic activity of HtrA2 rapidly increased with temperature, and it drastically decreased at and over 60 degrees C. Structural analysis using far-UV CD spectroscopy and gel filtration revealed no significant change in the secondary structure of HtrA2 from 25 to 70 degrees C, or in the oligomeric size between 25 and 55 degrees C. However, a significant change at the tertiary level, as examined using near-UV CD, was observed for HtrA2 in the range from 25 to 60 degrees C. Differential scanning calorimetry indicated that HtrA2 exhibits a thermal transition beginning at around 61 degrees C. The fluorescence intensity of ANS interacting with HtrA2 decreased with increasing temperature. HtrA2 was found to be able to complement DegP function at 44 degrees C, indicating that HtrA2 could have protective functions in mitochondria.