This paper introduces a dynamic systems based method for assessing the impacts of urban growth on transportation demand. Urban transportation system is a complex system and influenced by social, economic and environmental factors. It is considered that proper transportation planning can contribute significantly to the mission of urban sustainable development. However, there is little existing work to provide appropriate methods to explore the complex relationships between transportation system and urban growth. The study described in this paper introduces a simulation model, using a system dynamics principle, to explore the inter-dependent relationships between the various components. The components in this exploratory study include population, economy, housing, transport, and urban /developed land. The findings are expected to assist in the process of government's policy decision making concerning transportation planning and urban development. The model is calibrated and tested with data from the City of Toronto.