Based on the D0 -> K- mu+ nu and the D0 -> pi- mu+ nu samples obtained by the FOCUS photoproduction experiment at Fermilab, we measure the q^2 dependence of the f_{K}^{+}(q^2) using both parametric and non-parametric methods and compare the results with the latest unquenched Lattice QCD calculations. We also fit the form factors to the
pole dominance form and find a pole mass of 1.93 +- 0.05 +- 0.03 GeV for the D0 -> K- mu+ nu and 1.91^{+ 0.30}_{- 0.15} +- 0.07 GeV for the D0 -> pi- mu+ nu.