The grain boundary structure is usually described by the coincidence site lattice (CSL) model based on the misorientation of adjoining crystals. Therefore, the objective of the present investigation is to seek the correlation between CSL fraction and creep resistance of modified 9Cr-1Mo steel (P91) with and without boron addition. Results showed that CSL fraction increases with increase in heat treatment temperature and this increase is more prominent in boron containing modified 9Cr-1Mo steel. Creep test results show the increase in creep rupture life with increase in CSL fraction for both the base metals; but this increase is more in boron containing steel than the boron free steel. This improvement is attributed to the stability of CSL boundaries in the material. In spite of the increase in CSL boundaries with normalizing heat treatment temperature, boron free material shows less creep rupture life in its weldment than the boron containing steel weldment. © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license.