In this case study, a straight rectangular fin with insulated tip in a situation that its thermal conductivity coefficient is dependent upon the temperature has been analyzed in a simple and innovative method entitled "Akbari-Ganji's Method" or "AGM". Comparisons have been made between AGM and Numerical Solution and these results have been indicated that this approach is very efficient and easy, so it can be applie d for other nonlinear equations. It is necessary to mention that there are some valuable advantages in this way of solving differential equations and also the answer of various sets of complicated differential equations can be achieved by this method which in the other methods so far they have not had acceptable solutions. The reasons of selecting AGM for solving differential equations in a wide variety of fields is: not only in fluid mechanics but also in different fields of sciences such as solid mechanics, Vibrations, Strength of materials, Chemical engineering, etc. in comparison with the other methods are as follows: Without any dimensionless procedure, we can solve many differential equation(s), that is, differential equations are directly solvable by this method which means that it is not necessary to convert variables into new ones. According to the aforementioned assertions which will be proved in this case study, the process of solving nonlinear equation(s) will be very easy and convenient in comparison with the other methods