As capital structure is important to the company to generate its assets, operations and future growth and
finally result to maximize the valuation of firm. Thus, what is the determinant of firm capital structure is a basic
question of the study. For listed company in Thailand stock exchange market, the study confirms that leverage ratios
increase with firm size, and decrease with profitability significantly. Nonetheless, there are no significant
relationships between tangibility, growth opportunity, business risk and leverage ratios. In sum, the firm size and
profitability are significant determinants of capital structure in Thailand.