A goal of a study is to investigate how a zero-premium affects the enrollment decisions and plan choices of consumers in the ACA health insurance market, especially in the Marketplace. Using individual-level enrollment and claims data collected by the state-run Marketplace, the study also aims to analyze types of people reacting to zero-cost plans by age and number of specialists seen. This study could contribute to understand the health insurance demand at a specific price of zero and to provide an implication on the subsidy (premium discount) design in the regulated market, to have more people insured.. A goal of a study is to investigate how a zero-premium affects the enrollment decisions and plan choices of consumers in the ACA health insurance market, especially in the Marketplace. Using individual-level enrollment and claims data collected by the state-run Marketplace, the study also aims to analyze types of people reacting to zero-cost plans are and how the magnitudes of effects would vary by enrollee's age or income level. This study could contribute to understand the health insurance demand at a specific price of zero and to provide an implication on the subsidy (premium discount) design in the regulated market, to have more people insured. An identification strategy of the research mainly comes from a unique premium-linked subsidy structure in the Marketplace and the policy change made in 2017.