The study assessed the HIV treatment cascade of people living with HIV (PLWH) in Iran and the reasons for gaps in HIV services in 2014. We did the cascade analysis using the WHO HIV test-treat-retain cascade analysis tool (2014). To measure the gaps, we check the records for all PLWH reported to the national HIV surveillance till September 2014 to define how many were alive, linked to HIV care, assessed for CD4 count, started antiretroviral therapy (ART), and reported to be alive up to five years after ART. To assess the reasons for gaps in the cascade, we ran desk review, four focus group discussions (FGD) and fifty one in-depth interviews with key informants including PLWH. Spectrum model estimated 75,700 PLWH in Iran, of whom, only 22,647 people (%30) were diagnosed. Of those who were diagnosed with HIV, 16,266 people (%72) enrolled in HIV care. About 13% of all PLWH retained in HIV care and only 9% (6,745 people) had ever tested for CD4 count. Only 8% (6,018 people) of all PLWH received ART by September 2014. We found the biggest gap in the HIV treatment cascade to be HIV diagnosis as only 30% of PLWH have been diagnosed. Innovative strategies to improve HIV testing, particularly those work best for key populations, is critical to reach the 90–90-90 targets.