A node in Mobile Ad-hoc Network (MANET) solely depends on neighbor nodes for its connectivity to the outer networks. It is completely different with fixed network connection where a central infrastructure is providing connectivity to the outside network for all mobile nodes there. This kind of situation makes MANET easier to build rather than fixed network with certain infrastructure. However, this nature of MANET makes it very vulnerable to various attacks, especially by nodes within the MANET that is called malicious nodes. This paper provides a preliminary result for MANET security enhancement based on AODV-UI routing protocol. In this work we implement an algorithm to detect and remove malicious nodes in AODV-UI routing protocol. We evaluate our work in different scenarios by varying the number of nodes, the number of malicious node, the sending rate of the node in concern, and the type of the attack ie route poisoning, black hole, packet spoofing. Our experiment shows that on average, an attack can be completely removed within 0.48 seconds in the worst case, with the traffic rate of 100 kbps, and 0.04 seconds in the best case, with the sending rate of 10 kbps.