Instability indices are the parameters that could be used to determine instability in the different parts of the atmosphere. In this study, we have used some of the indices to determine the probability of rainfall for cloud seeding operation in Mashhad region of Iran during (1993-2010). Used indices are including Precipitation Water of Cloud (PWC), Showalter Index (SI), K Index (KI) and Convective Available Potential Energy (CAPE). The optimal threshold of specific index for cloud seeding decision has been estimated using the best fitted curve on scattering diagram of the rainfall data and the index data. The optimal thresholds of PWC, SI and KI for convective cloud seeding activities were found 10 mm, 2�C and 24�C, respectively. The results show that rainfall could be occur with different amount of CAPE, even zero. Therefore, we could not consider any threshold for this index. The rainfall increase with increasing PWC, KI and CAPE; and decreasing SI in the region.