In this era of wireless devices, Mobile Ad-hoc Network (MANET) has become an indivisible part for communication for mobile devices. Therefore, interest in research of Mobile Ad-hoc Network has been growing since last few years. In this paper we have discussed GRAY Hole attack in OLSR routing protocols in MANET. Security is a big issue in MANETs as they are infrastructure-less and autonomous. Main objective of writing this paper is to apply gray Hole attack in MANET& know How its effect on the MANET Environment. This article would be a great help for the people conducting research on real world problems in MANET security