The light- and thermal-induced changes of Si-H bonds in undoped a-Si:H have been studied on the infrared absorption of Si-H stretching mode, using the infrared photothermal deflection spectroscopy and the photothermal bending spectroscopy. The results show that the IR absorption of Si-H bonds increases by light soaking of the high power similar to 500 mW/cm(2). The change of IR absorption of Si-H bonds reoccurs by thermal annealing at 200 degrees C. This change is related to the increase of the dangling bonds under light soaking. Furthermore, we observe the change of the elasticity modulus by light soaking, using the photothermal bending spectroscopy. The structural change in a-Si:H is discussed based on these results.