Structures of as-prepared and annealed porous silicon samples have been characterized by Si-29 nuclear magnetic resonance (NMR) spectroscopy. Magnetic shielding tensors have also been calculated using ab initio molecular orbital calculations to help the characterization. After the annealing at 300 degrees C for 4 h, the NMR peak position shifted slightly and the chemical shia anisotropy increased. Hydrogen desorption during the annealing gives (SiH)(2) dimer structure from SiH2 configuration. The assignments of NMR signals are as follows: -94 ppm due to SM, -85 ppm due to SiH2 and about -85 ppm due to (SiH)(2). The (SiH)(2) dimer has large chemical shift anisotropy and small magnetic shielding, which are caused by the strained conformation of the dimer.