Experimental studies by means of a constant photocurrent method (CPM) have been carried out on the deep defect states in undoped hydrogenated amorphous silicon (a‐Si:H). Assuming Gaussian energy distributions, two types of defect states (ST1 and ST2) have been found from careful analysis of CPM spectra; one of these states (ST1) is a neutral Si dangling‐bond (SiD0) state, and the other (ST2) is a negatively charged dangling‐bond‐like defect state located in the lower gap. ST2 lies deeper in energy by 0.1 eV, and has a narrower full width at half‐maximum (FWHM) when compared with ST1. Possible candidates for ST2 have been discussed by referring to the previous models of defects ina‐Si:H. The FWHM of ST1 (the SiD0state) does not always depend on the overall structural disorder estimated from Raman spectra. The energy position of the SiD0state measured from the valence‐band edge is almost independent of the optical band gap. Long exposure to light increases the density of ST1 (the SiD0state) and decreases that of ST2, which suggests that defect conversion proceeds during the illumination.