The fundamental crrar is considered as one of the vice of consent which makes the contract void under the English common law, and voidable according to the principles of equity. It is worth-bearing in mind that the operative fundamental mistake is classified into four main subtypes: the first is the identical or common mistake. The second is the non-identical mistake which arises either from the Cross-Purposes between the contracting parties, and known as Mutual Mistake. Or from one of them, while the other knows or is presumed to know the truth of the mistake, and referred to as a Unilateral Mistake. The third is the mistake as to the Identity of the contracting Party. And the fourth is the mistake as to the Contractual Document. Whereas the Iraqi Civil law No. (40) of 1951 has also adopted the fundamental mistake which must be so severe that the mistaken contracting party would have not abstained from contracting, had he understood the truth and not fallen in it. Whether it be on a fundamental characteristic of the subject-matter of the contract, or on the Identity of the contracting Party, or on affairs which the impartiality of dealings consider as fundamental elements of contracting.