This paper focuses on the formation of contract under CISG through the case by the Korean court. Under the CISG, an offer means that a proposal for concluding a contact constitutes the offer, if it is sufficiently definite and indicates the intention of the offeror to be bound in case of acceptance. An acceptance is statement made by or other conduct of the offeree indicating assent to an offer, and this statement purports to be an acceptance but contains additional or different terms which do not materially alter the terms of the offer constitutes an acceptance. In practice, parties negotiate for lots of contract terms to conclude the contract, and the last reply indicating of assent which is made by statement or other conduct to a proposal for concluding a contract would be an acceptance. At this time the most important factor is the intention of parties whether they intend the offer or the acceptance, Purchase order by buyer or Offer Sheet by seller would be understand as condition precedent for the formation of contract. Nevertheless, keep in mind that the Korean court is consistent in the way Purchase order by buyer or Offer Sheet by seller is an acceptance to conclude contract.