Commercial instruments, in particular meaning, or bills are one of the most important means for business parties to enter into transactions. Such instruments are considered as a certain form of property with a great variety of uses. For example, they can be used as a means to pay or to use as a guarantee or security. But a question has always been posed: “whether such instruments can be used as security or guarantee in accordance with law”. Mortgaging the instruments which are not regarded legally to have genuine value is under question and discussion among law professionals. Some of them absolutely accept this and some not. A group of them discuss the instruments stressing on the fact that they are bearing or not. Most of the law professionals consider such instruments within the framework of mortgage contracts, and they place no difference between security/guarantee and mortgage; while, it seems that mortgage has a general and broad meaning, while mortgage is a particular example of that and is subject to civil code. Therefore, it can be said that mortgage is a security, but the contrary does not hold. This separation occurs in British law as well.Here in this paper, I deal with the legality of mortgage and security uses of bills in Iranian and British laws. General conclusion which can be drawn from my research is that bill means check and promissory note which is a certain kind of credit property of movable nature, so it is capable to be legally transferred, received, mortgaged and used as security or guarantee. Such matters have not also been accepted by British law, and there can be no difference between Iranian and British laws in this regard, although the mortgage is understood and treated differently in the two laws.