With the emergence of the sharing economy, the Mobility-on-Demand service becomes a complement to traditional urban transportation. To alleviate the urban traffic congestion as well as slowing global warming, the government should encourage the use of public transit to reduce the number of vehicles on the road and their carbon emission. However, the existence of the first-mile and last-mile problems creates barriers for commuters to access their local public transit. This paper attempts to solve these problems by designing a multimodal transportation network that integrates a Mobility-on-Demand service and the public transit system. This MoD service connects commuters between their home or destinations with nearby transit stations, and is op- erated by the local government. In this paper, we discuss an optimization problem that tries to find the optimal service radius of the MoD service for each service zone that maximizes the aggregate number of commuters using public transportation. We also provide an empirical case study of specific districts of New York City to show that the introduction of the Mobility- on-Demand service could efficiently reduce the number of vehicles on the road.