The purpose of the paper is to investigate the global existence of solutions to initial value problems for nonlinear fractional differential equations on the semi-axis. More precisely, it deals with the initial value problemwhere 0 < α < 1, D α 0+ denotes the Riemann-Liouville fractional derivative of order α, and f : (0, ∞) × R → R is a continuous function. Unlike all the previous papers dealing with the problem of existence of solutions to (*), this problem is solved here by constructing a special locally convex space which is metrizable and complete. Then Schauder's fixed point theorem enables to provide sufficient conditions on f , ensuring that (*) possesses at least one solution. The growth conditions imposed to f are weaker than other similar conditions already used in the literature.MSC 2010 : Primary 34A08; Secondary 26A33, 34A12