Aquifer parameters, transmissivity(T) and storage coefficient(S), and the well losses, mostly have been estimated in practice independently from each other. This independent estimation means that pumping well is assumed to be 100% efficient in determination of aquifer parameters. On the other hand; drawdowns, observed during the course of a pumping test, are strictly related to the well losses as well as the aquifer parameters. This interrelation, implies the necessity of simultaneous determination of both aquifer parameters and the well losses. Thus in this study, a computer program is written for this purpose using nonlinear curve fitting techniques and, it has been shown that estimated aquifer parameters may be erroneous if the well losses is not taken into account.