Topological defects are a class of important concepts in particle cosmology. They are physical objects formed when a fundamental symmetry is spontaneously broken during the cooling of the expanding Universe. Typical topological defects include monopoles, domain walls and cosmic strings.  While the first two lead to a serous cosmological problem and should be diluted by inflation, the last one is safe and have some interesting features favoured by theorists. The most important one is that it is a main source to generate cosmic background of stochastic gravitational waves (GWs). An observation of this background in the future will be helpful for us to construct a fundamental particle theory at super-high energy scale. On the other hand, biased domain walls could avoid the cosmological problem and provide another source for GWs. In this paper, I will introduce mechanisms of GWs from topological defects and discuss their connections to fundamental particle theories.