Even though Human Immunodeficiency Virus (HIV) can infect Old World Monkey (i.e African Green Monkey) cells, replication of the virus is hampered with different kinds of restriction mechanisms before the integration of viral genome to host genome. After viral integration to host genome, HIV-1 TAT protein is expressed, which plays an important role for the generation of other proteins for viral production. Previous studies which were performed in our laboratory indicate that Secretory Leukocytes Protease Inhibitor (SLPI) overexpressed in existence of HIV-1 TAT protein. This result was confirmed with 2D-PAGE, qRT-PCR and Western Blot Analysis. The aim of this study is to understand how Vero Stable Cell Lines which produces HIV-1 TAT protein can affect expression levels of SLPI. At mRNA level of SLPI expression was confirmed with qRTPCR. In order to measure SLPI expression at the protein level in the presence of HIV-1 TAT,Western Blot and Sandwich Elisa methods were used. Luciferase Assay process was performed in order to measure the effects of SLPI on HIV-LTR. Consequently, expression of SLPI in Vero Stable Cell Line increased in the existence of HIV-1 TAT protein. Additionally, SLPI inhibits HIV-LTR function through suppressing NF- κB promoter which exists in HIV-LTR.