Online transaction security is basically the protection of online transaction assets from unauthorized access, use, alteration or destruction i.e. to ensure CIA (Confidentiality, Integrity and Availability) triad of information security. Online transactions are emerging as very useful to end users and business parties, but it also creates a set of new risks and vulnerabilities such as security threats. Information security, therefore, is an essential requirement for efficient and effective transaction activities over the internet. In this paper, an overview of online transactions and online transaction security, purpose of security in online transactions, different security issues in online transactions and various security mechanisms of online transaction system has being discussed.