For many purposes the natural-synthetic fiber-blend fabrics are more suitable than pure natural or synthetic products. It is often possible to obtain a maximum in clothing and textile technical properties by compensating the defects of one fiber by using an other totally different fiber. Many problems, however, have arisen in the production of flame-retardent fabrics because the use of synthetic fibers often makes the fire retardancy less effective. In our 2-year research project different fire-retardant (FR) viscose–polyesters fabrics were prapared at first in the laboratory scale. The natural type raw materials were Modal Prima viscose and normal FR–viscose cotton type staple fibers. The synthetic raw materials were FR–polyesters of the same type with two different flame retardants. Test fabrics were knitted in the laboratory by using seven blended yarns in the ratios 100/0, 80/20, 65/35, and 50/50 and vice versa. Cotton type PVC–fiber was also used in some experiments. All these test fabrics were also finished chemically by using normal crease-resistant (DMU, DMEU, DMDHEU, and TMM) and flameretardant (N,-methylolphosphonopropionamide and THPC) finishing chemicals. The textile and fire-retardant properties of the original and finished fabrics were estimated by using addon, tensile strength, LOI-value, and vertical flame test determinations. The mechanism of flame retardancy was also studied with DSC technique, P- and N-analysis and char investigations. The test results of viscose/polyester studies were compared with the results of cotton/polyester studies. After laboratory studies the best methods for FR–viscose/polyester fabric production were chosen, and the fabrics were manufactured. The fabrics were home-washed 20–50 times, and the textile and FR-properties were determined after each 10 washings. These results were again compared with results of cotton/polyester fabrics.