ZDDP is known to protect adhesion wear by forming reaction film by direct contact between sliding two substrates. However, it is known to raise the friction coefficient even though its anti-wear properties. Therefore, to reveal the mechanism to raise the friction, much research has been conducted to focus on the relationship between the formation and the physical property of ZDDP tribofilm. In this study, the purpose is to reveal the friction mechanism of ZDDP tribofilm, and we investigated the physical properties of ZDDP tribofilm during in situ AFM observation. As a result, it is shown that the ZDDP tribofilm was gradually forming as increasing sliding cycles and, regardless of increasing tribofilm thickness, the friction coefficient reaches the stable value after 400 cycles. In addition, we confirmed that the adhesion force on the sliding surface increases by forming the tribofilm. Therefore, these results show that ZDDP raises the friction by forming the high adhesion tribofilm.