We have investigated the influence of a magnetic field on YBa2CU3O7 (YBCO) films grown directly on polycrystalline silver substrates by metalorganic chemical vapor deposition (CVD). Clearly, application of a magnetic field during the CVD process not only changed the surface morphology of YBCO films, but also improved their crystallinity and grain connectivity. Furthermore, it is found that the growth mechanism of YBCO films was changed from the spiral growth mode at zero field to the 2D or 3D island growth mode under an external magnetic field. A discussion on the mechanism of this field-induced growth mode change is also presented.