Abstarct: In this thesis, the influence of major ions of seawater on MNB concrete was investigated experimentally. A mixture of water-to-cement ratio of 0.61 was used to make concrete samples. The experimental settings included sixty six (66) concrete cubes of 15 cm to measure the compression strength, six (6) concrete beams of 10 * 10 * 33 cm to measure the flexural strength and six concrete cylinder of 15 * 30 cm to measure the tensile. All tests were performed with triple samples. The samples were cured at 1, 7, 28, 90 and 150 days and then their compression, tensile and flexural strengths were tested. The results showed that compression strength of MNB concrete cured in sea water at ages 7 and 28 days increased like other concretes cured in sea water but unlike other concretes their long compression strength won't be decreased with curing in sea water. The 28- days- flexural strength and 1-day- tensile strength of MNB concrete cured in sea water was increased about 6% and 70 % respectively. The compression strength of specimens made with combination of sea water and MNB was not so different with specimens made with MNB.