The thesis deals with the legal system of guarantee letter in Omani law. A competitive study between Omani, Egyptian, UAE, and Kuwaiti laws included with the main concept of the letter of guarantee. Types of letters of guarantee, rights and obligations arising out of letter of guarantee, requirements of issuance of letter of guarantee, possibility of extension of period of letter of guarantee, attachment of the amount of letter of guarantee and finally termination of letter of guarantee. The main purpose of this research is to highlight the particularity of the letter of guarantee in Omani law. The practical issues of letter of guarantee in Omani banking system. The research showed that the Omani legislator regulates the legal system of letter of guarantee according to the most general rules in this field. Comparing Omani rules of letter of guarantee with similar rules in other Arab laws, it is clear that the Omani legislator has ignored to regulate some issues. The results of the research indicate to these issues with a recommendation to modify the Omani rules of letter of guarantee to regulate these ignored issued including: To add an article entitle the issuer of letter of guarantee to attach the amount of the letter of guarantee due to serious reasons. - To add an article entitle the bank to release the security of the letter of guarantee, in case the beneficiary doesn't claim the bank to pay the amount of the guarantee letter