IAS 39 has been described as being the most difficult standard to be implemented and understand The main aims of this research were to examine the effect of the implementation of IAS 39 on the investment's decisions of individuals and institutions in Jordan; to evaluate the understanding of IAS 39 by Jordanian investors and the effect of their understanding upon their investment decisions; and to examine the effect of some factors such as the degree of knowledge and the nature (individual or institution) of the investor on the degree of understanding of this IAS because. In the current study, Firstly, the results supported the first hypothesis that the implementation of IAS 39 would affect the reported profits of companies and, consequently, Jordanian investors would decrease their investment in these companies. Secondly, no significant differences were found between different fields of education, different professional certificates, and different years of experience with regards to the understanding of the implementation of IAS 39 and its effect; but, statistical differences were found between different levels of education at the 5% level, and between individual and institutional investors at the 10% level.