In Mobile Adhoc Network the design of security model is a very critical and essential topic, this is due to the rapidly changing nature of this type of networks. Many other factors should be in consideration while handling mobile adhoc networks like the lake of infrastructure, bandwidth, etc. a security scheme is proposed based on Public Key infrastructure for distributing session keys between nodes. The length of those keys is decided using fuzzy logic manipulation for the discrimination between some of the attacks applied over this kind of networks. The proposed algorithm of Security-model is an adaptive fuzzy logic based algorithm that can adapt itself with the dynamic conditions of mobile hosts. Experimental results show that the use of fuzzy based security can enhance the security of mobile adhoc networks.