Performance Analysis of TSDRP and AODV Routing Protocol under Black Hole Attacks in MANETs by Varying Network Size Mobile Ad hoc Network (MANET) is a self-configured collection of mobile nodes in which there is no need of predefined infrastructure and centralized authority. MANET is particularly vulnerable to security attacks due to its fundamental nature. Ad hoc On-demand Distance Vector (AODV) is a very popular routing protocol of MANETs, but it has no security measures in-built and vulnerable to many types of attacks. Black hole attack at network layer is the most attention seeking attack in AODV routingprotocol. This paper study the impact of network size of our propose Trust Based Secure On Demand Routing Protocol called “TSDRP” and AODV routing protocol for making it secure to thwart Black hole attack. To evaluate the performances, we have considered Packet Delivery Fraction (PDF), Average End-to-End Delay (AED), Average Throughput (AT) and Normalized Routing Load (NRL).