  A new approach, based on reciprocal space vectors (RSVs), is developed to
determine Bravais lattice types and accurate lattice parameters of epitaxial
thin films by high-resolution X-ray diffractometry (HR-XRD). The lattice
parameters of single crystal substrates are employed as references to correct
the systematic experimental errors of RSVs of thin films. The general procedure
is summarized, involving correction of RSVs, derivation of raw unit cell,
subsequent conversion to the Niggli unit cell and the Bravais unit cell by
matrix calculation.Two methods of this procedure are described in 3D reciprocal
space and 6D G6-space, respectively. The estimation of standard error in the
lattice parameters derived by this new approach is discussed. The whole
approach is illustrated by examples of experimental data. The error of the best
result is 0.0006 {\AA} for the lattice parameter of ITO (Indium tin oxide)
film. This new RSV method provides a practical and concise route to crystal
structure study of epitaxial thin films, which could also be applied to the
investigation of surface and interface structures.
