The effect of a flow guide in a vertical MOCVD reactor on the deposition uniformity and growth rate of thin YBCO films has been studied. Without the flow guide the growth rates are low, have a poor uniformity and the film composition is not stoichiometric. The growth rate of the films grown using a reactor with the flow guide was approximately twice that without the flow guide. Using this flow guide the growth rates were 0.4–0.7 μm/h for growth temperatures varying between 600 and 750 °C, and the crystalline quality as well as the surface morphology of YBCO films on MgO substrates is improved. For films grown at temperatures above 650 °C the composition of Y:Ba:Cu is 1:2:3, as confirmed by EDAX spectra. Films deposited without and with the flow guide at 700 °C have critical temperatures around 85 and 88 K, respectively. The reduction in ΔwTc (Tc,zero—Tc,onset) also shows an improvement of the superconducting properties of YBCO thin films deposited with a flow guide.