Consensus mechanism, as a fault-tolerant mechanism, is the core foundation as well as crucial guarantee of the security of blockchain. Blockchain (what is blockchain) is a decentralized system which relies on consensus mechanism to achieve the agreement on block generation among tens of thousands of nodes around the world. There are various consensus mechanism out there, Proof of work is the most well-known one adopted by the first cryptocurrency, Bitcoin. What is Proof of Work? Proof of work is described as a system allowing every node to work out difficult hash puzzles with large amount of computer calculations. In the bitcoin network, a distributed ledger system, nodes/miners figure out complex mathematical equations so as to process and validate the BTC transactions, which is also called mining. The first node work out the solution of the hash/mathematical puzzle will be the block generator and get rewards. With POW, cyber-attacks which aims at exhausting the resources of a computer system, can be deterred. Pros & Cons of Proof of Work1. POW algorithm is easy to accomplish and the threshold is low for nodes. 2.Protect blockchain system from DDos-attacts (denial-of-service) which sending multiple malicious and fake requests. 1. Energy consumption and huge cost. Specialized computer hardware is required to calculate complex hash puzzles, which consumes a lot of energy and leads to expensive energy charges. 2. In case the 51% computing power/mining hashrate is controlled by a group of miners, they can control the whole network to prevent new transactions being confirmed and double-spend the cryptocurrency. Due to the disadvantages of Proof of work, many consensus mechanism such s POS, DPOS, etc. get on the stage of blockchain as well as cryptocurrency. And which one will be the most useful one, let's stay tuned.