Positron annihilation lifetime spectroscopy (PALS) measurements of SmFeAsO polycrystalline sample were first carried out at 298 K. Two lifetime components of 1516 ps and 2903 ps were obtained. According to two-state capture model, the free annihilation lifetime in polycrystalline SmFeAsO is 1870 ps, which agrees well with the calculated value（in general gradient approximation） of 173 ps in SmFeAsO single crystal. Superposed-neutral-atom model and the finite-difference method (SNA-FD) were used to calculate the positron annihilation information in single-crystal SmFeAsO. The results show that the ratio of annihilation rate with valence electrons to annihilation rate with core electrons is 106, while the ratio of annihilation rate with electrons from Fe, As, Sm, and O atoms are 1∶13∶12∶1