The mechanical properties of hybrid (commingled) yam based reinforced thermoplastics depend on many parameters of the fiber components as well as of the different technological conditions during the commingling and the hot press molding. Among the fiber parameters the diameter ratio of the matrix and the reinforcing yarn components are most important to obtain best mechanical properties of the reinforced thermoplastics. The paper treats theoretical considerations in order to reach a high probability of coating each reinforcing filament with the thermoplastic material during the hot press molding. The calculations include the filament ratios and the achieved volume ratios of both hybrid yarn components of the finished reinforced thermoplastics. It is demonstrated that under different optimization criterias (same filament numbers for both components or the same surfaces of the reinforcing yarn filaments and the matrix yam filaments before the hot press molding) a more or less small range of filament ratios seem to be valid for manufacturing a homogeneous faultless reinforced thermoplastic. Quantitative calculations for two hybrid yarn systems PEEK/carbon fibers and PP/glass fibers, presented in diagrams, are demonstrated. Technological conditions for the spinning of micro fibers like PEEK-filaments complete the presentation.