This research provides an empirical analysis of the factors influencing the strength of labor unions in Thailand in order to provide long-term implications for labor unions and labor-management relations in Thailand and other countries. The union's presidents, board committees, and members of labor unions in nine private companies and two state enterprises were participated in this survey. Based on 1761 valid responses from a total population of 44573 persons, our findings demonstrated that 13 factors tend to affect the strength of labor unions in companies and state enterprises in Thailand. Represented in descending order of influence, the macro-level (or national-level) factors include labor union association; mass media; cultural factors; labor relations law; economic, political, and population changes; and the role of government. The micro-level (or firm-level) factors in descending order of importance success in collective bargaining; labor union policies; union member unity; employer policies; financial status of labor union; union president and board committees; and the relationship between labor union and employer. [PUBLICATION ABSTRACT]