Background: Estimation of the size of hidden and hard-to-reach sub-populations, such as drug-abusers, is a very important but difficult task. Network scale up (NSU) is one of the indirect size estimation techniques, which relies on the frequency of people belonging to a sub-population of interest among the social network of a random sample of the general population. In this study, we estimated the social network size of Kermanian males (C) as one of the main prerequisites for using NSU. Methods: A 500 random sample of Kermanian males between 18 and 45 years old were interviewed. We asked the size of their active networks using direct questions. In addition, we received the frequency of six names from the vital registry office among Kermanian males, and we estimated C indirectly using the received frequencies and the frequency of these names among the networks of our sample. Findings: Although different methods showed quite different Cs between 100 and 350, the best estimation for C was 303, which means that on average each Kermanian male knows around 303 males between the age range of 18 and 45 years. The estimated C did not have any strong association with the demographic variables of our subjects. Conclusion: Using the estimated C we may use the NSU technique to assess the frequency of many important hidden sub-populations such as drug-abusers and those who have sexual contact with men and women. Keywords: Size estimation, Social network, Networking, Addiction, Hidden population, Hard to reach population.