This chapter discuss the legality of smart contract in a decentralized autonomous organization (DAO). The regulation framework of blockchain is developing rapidly with many countries such as Malta and Estonia utilizing and allowing the use of blockchain. While many researchers have discussed the legality of smart contract, its relation to DAO as one of blockchain's applications is rarely discussed. There are three issues discussed in this chapter: the definition of DAO, the definition of smart contract, and the legality of smart contract in DAO. Each country has their own legal threshold of a contract's legality, but there are generally five elements in a legal contract, namely offer and acceptance, consideration, intention, certainty, and completeness. It is possible for the smart contract to fulfil the five legal elements of a contract. As there are different regulations in different countries, there may have been different elements to the law. In conclusion, the legality of smart contracts, like blockchain itself, depends on who uses it, where it is used, and how it is used.