This paper investigates the relationship on stabilizability of linear time-invariant (LTI) systems by P and PI controllers. Elementary tools such as the Routh stability criterion and Root-Locus method are employed in the analysis. It is found that PI can stabilize all the systems that P stabilizes but in general the converse is not true. The cases with the equivalence of stabilizability by P and PI are established and they are in general loworder systems with few zeros. The cases with non-equivalence are also identified.