The tensile properties of high-strength poly(vinyl alcohol) (PVA) fibers after heat treatment in air, water, and engine oil were studied. The results show that heat treatment in air, water, and engine oil have a different influence on the tensile properties of high-strength PVA fibers. After heat treatment in air, the fibers possess excellent heat stability of the tensile properties. But in water, especially in hot water, the tenacity, Young's modulus, and specific work of rupture of the fibers decrease, while the elongation at break of the fibers increases. Similarly, engine oil has a significant influence on the tensile properties of the fibers. When the temperature of engine oil is above 120 degrees C, the tensile properties of the fibers decrease drastically. We also discuss the influence of heat, water, and engine oil on the tensile properties of high-strength PVA fibers in relation to the structure of the fibers.