Postsecondary institutions, especially selective ones, use standardized tests such as SAT and ACT as important indicators to measure applicants' academic abilities. However, many studies have found that the test scores correlate to applicants' racial/ethnic backgrounds. Selective institutions work hard to secure their diverse classes. Hence, more selective institutions expect to improve diversity by implementing the test-optional policy. Can this policy meet their expectations in terms of diversity? The purpose of this dissertation is to examine the effects of the test-optional policy on the structural diversity of selective postsecondary institutions. This dissertation uses quasi-experimental techniques to assess the relationship between test-optional policy implementation and the representations of various racial/ethnic groups among the 510 selective institutions in Barron's Ranking. The findings show the connections between test-optional policy and structural diversity in different institution types and selectivity levels. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]