Abstract: Blockchain is one of the most trending technologies which plays a major role in online transactions using cryptocurrencies. The blockchain is a chain of blocks that consists of allthe transactions up to the size of 1MB. This blockchain has many properties such as decentralization, immutability, transparency, and audibility, making transactions more secure and tamper-proof. It is tamper-proof because there is no possibility to tampera block as every block in the blockchain has the hash of the previous block. And among cryptocurrencies, bitcoin is one ofthe most popular... In fact, blockchain was introduced to theworld because of bitcoins. Apart from cryptocurrency, blockchain technology can be used in financial, NYC, and social services, risk management, food, healthcare facilities, and so on. Numerous studies have examined the potential that blockchain offers inmultiple application sectors, as well as the benefits and different kinds of blockchain. This paper presents a comparative studyof bitcoins in blockchain technology and the workflow of thebitcoin, compares bitcoin and Ethereum, and provides the use of smart contracts in this emerging technology. The methodology considered for this research is the Ethereum blockchain in smart contracts using a solidity programming language.