In the current paper a new method is presented that determines the five macroscopic parameters of a grain boundary (GB) from electron diffraction patterns and bright field (BF) images in a transmission electron microscope. Usefulness of the method is demonstrated on a set of GBs measured in laser crystallized Si thin films. To characterize the geometry of a grain boundary, we have to determine the misorientation between the neighboring grains, and the direction of the GB-plane. The misorientation is calculated from two convergent beam electron diffraction (CBED) patterns taken on the neighboring grains, and the plane-normal can be calculated from bright field (BF) images of the GB. The thickness of the sample is also needed and it is measured from a CBED pattern taken in two-beam condition. It has been previously observed in metallic thin films, that the GBs can minimize their energy in two alternative ways in thin films: either forming a GB plane with low energy density or minimizing the area of the plane. In the last case the GB plane is almost normal to the surface of the specimen and has a general index. We found, that boundaries with exact Σ3 misorientation generally adopted low energy-density {111} boundary planes and formed coherent twins. The rest of the boundaries adopted general index planes and minimized the surface area of the boundary. The last group included boundaries with small deviation from Σ3 misorientation, other special highangle boundaries with Σ-value>3 and boundaries with general misorientation.