In this paper, we give some results on closed polynomials and factorially closed polynomial in n variables which are generalizations of results in [7], [12] and [13]. In particular, we give a characterization of factorially closed polynomials in n variables over an algebraically closed field for any characteristic. Furthermore, as an application of results on closed polynomials, we determine kernels of non-zero monomial derivations on the polynomial ring in two variables over a UFD. Finally, by using this result and the argument in [15, §5], for a field k, we determine the non-zero monomial derivations D on k[x, y] such that the quotient field of the kernel of D is not equal to the kernel of D in k(x, y).