Currently available statistical methods that have been developed to asses the uncertainties and sensitivities of large complex computer codes have many drawbacks and constraints when the cost and the time to run the codes are important factors in particular. A new method entitled "Iterative Stagewise Group Uncertainty Analysis Method" is developed which uses the underdetermined set of equations and group screening design method. This method uses the two level resolution III design. Analysis of uncertainty and screening can be done simultaneously using newly defined variance importance. This method is flexible to use according to the characteristics of the problem. Comparisons with Plackett-Burman Design and Monte Carlo Method showed that the results obtained from the developed method in the present work is quite reasonable and satisfactory.