String theory is believed to be a promising candidate for a theory of everything. There are five consistent superstring theories in 10-dimensional spacetime. To make contact with the real world particle physics, the six extra dimensions have to be so tiny that they are undetectable. This is called the string theory compactification. In the past, we have studied the structure of D9-brane Chan-Paton factors in Type II orientifold theories. This provides important information on string theory compactification. I am now working on the string theory phenomenological model building.