Using the computer model of the Upper Bukit Timah Basin of Singapore, the effect of urbanized land condition on the flood peak increase due to urbanization has been assessed. The model results are presented in isopleths, which are lines of equal ratio of flood peak after and before urbanization. The degree of urbanization is expressed in terms of the percentage of area developed and the percentage of area channelized. By subjecting the Basin to the Singapore 2-year rainfall, the results show that the transformations from a forest to a developed land with 30% imperviousness may cause a three-fold increase in the flood peak. If the imperviousness of the developed land were to increase to 100%, the flood peak increase could be as high as five times. Evaluation of flood peak increase is therefore highly dependent on the degree of imperviousness of the urbanized land.