Semantic completeness and syntax completeness are important characters of the general formal systems, two the completeness have different meaning for a formal system. This paper analyze the relation between semantic completeness of a formal system S and syntax completeness under certain condition and the relation between the syntax completeness of S and the syntax completeness of consistent extension of S, and following results are proved. (1) If S has syntax completeness then S must has semantic completeness when S has soundness. (2) If S doesn’t has syntax completeness then any consistent extensions of S don’t have syntax completeness either, the converse not holds. (3) Any extensions of the first order predicate logic don’t have syntax completeness.