Because charter school principals are granted more autonomy and discretion than principals of traditional public schools, it is imperative to search for the attributes of principals that may improve charter school performance. This study examines the relationship between principals' collective bargaining and charter school effectiveness. Using propensity score matching based on the School and Staffing Survey, I find that principals' unionization is positively and significantly associated with charter school performance. I also identify potential mechanisms through which principals' unionization influences charter schools: higher pay and better working conditions for principals, more formal principal evaluation, and more informal teacher evaluation.