Estimates of the number of individuals among key populations at higher risk for HIV exposure (KP) are essential to advocate for resources, prioritize prevention and other programmatic activities among groups, predict the number of persons living with HIV (PLHIV) and the impact of the HIV epidemic through modeling, and assess program coverage. Population size estimates (PSEs) are an important component of surveillance as well as monitoring and evaluation. Ministry of Health has started the population size estimation (PSE) since 2002. The PSE methods were kept updated and being improved for obtaining better result. Report of Size Estimates HIV of Key Population in 2016 is a renewal of the report of Size Estimation of Key Affected Population in 2012 issued by the Ministry of Health in year of 2014. This report describes a comprehensive situation and can understand in relation to the size of key affected population up to district level. The estimation results in 2012 showed that there were 7.4 to 10.2 million people with a median value around 8.8 million key population. The results of these estimates were then included in the calculation of estimation and projections of HIV/AIDS in Indonesia in 2011-2016. The 2016 PSE exercise was resulting final population size estimate of FSW, MSM, Waria, PWID, and Clients of FSW and Waria which the highest size estimate is clients of FSW which account for 5 million, and the smallest size estimates is PWID which account only 33 thousand. The results of these estimates are also then included in the calculation of estimation and projections of HIV/AIDS in Indonesia in 2015-2020. The process of size estimation of the key population has gone through a long and complex process involving a variety of associated partners. This estimation methodology and results have been reviewed by a group of experts and presented to stakeholders. The results of the reviews stated that with all the limitations that exist in the calculation of this estimate, the result is the best result that can be obtained with the data available at the time the calculation is done.