An oblique lattice system (OLS) is defined by two groups of parallel lines in a rectangle, and some properties of the system are mentioned. From the property of OLS that are constrained to rectangle sizes, it is shown that the system provides good mathematical treatments of lattices. Next, a simple algorithm to design integral lattice-based halftone masks by the OLS is introduced. In the algorithm, the "uniform balanced numbering" concept for general halftone masks is decomposed into a simple "local and global numberings" concept for integral lattice-based masks. The OLS is used to realize the local and global numberings concept and to know conditions of halftone masks to design. As a result, we can easily achieve halftone masks based on oblique lattices that realize the required conditions (mask size, resolution, line angle, etc.) and good image quality.