Online Banking offers customers the accessibility of managing one’s funds anytime, anywhere. However, any online transactions will be prone to security threats. Existing system uses two way authentication factor(OTP) that are easily cracked by cyber attackers leading to customers having their account details compromised while not having the provision of attacker’s identification. Therefore in this paper we propose mobile SIM serial based verification system to secure mobile transactions on mobile devices with Blockchain based server side secure system. The subscriber identity module(sim) serial number is registered with the user’s account and in case an intruder tries to initiate a transaction from some other sim, there arises a mismatch of the login credentials then system sends location information of the intruder to the bank. The bank then alerts the registered user by mailing them the intruder’s details. Since the mobile sim serial number is unique to the sim and is not provided manually rather extracted, there is no possibility of intruder initiating the transaction from other device, thus overcoming the concerns related to OTP. The Ethereum Blockchain technology provides server side database security by verifying the digital signatures during the transaction and consensus algorithm for transaction confirmation. Blockchain based security is mathematically proved for secure Banking transaction.