Let K/Fq be a global function field over the finite field Fq,and l be a prime number different from the characteristic of K. Denote by ζl a primitive l-th root of unity in a fixed algebraic closure of K. For two given elements a,b ∈ K*-(K*)l, we study in this paper the properties of radical extensions K(l√a) and K(l√a,l√b) of K. By the Kummer theory, we give a necessary and sufficient condition for K(l√a)/K and K(l√a,l√b)/K being not geometric extensions. Suppose that a,b ∈ K* - (K*)l are l-independent. For a prime divisor P of K and the corresponding discrete valuation ring OP, a necessary and sufficient condition for a,b,generating cyclic group (OP/P)* is presented by the properties of the above two function fields extensions. With the help of results obtained, the Dirichlet density of Ma,b, which is the set of prime divisors of K such that cyclic group (OP/P)* can be generated by a,b, is given explicitly in this paper.