The aim of this publication is to show the process of computer aided designing of the spur gear pairs having normal and modified straight teeth. For designing, the determination of the geometrical parameters is needed. Computer programs have been developed with which gear pairs having normal and modified teeth, with any arbitrary parameters can be analysed, designed and modelled. With these programs gear pairs having concrete geometry have been designed and CAD models have also been drawn to have further meshing, geometrical and TCA (Tooth Contact Analysis).