Flax stem is a source of fiber used by the textile industry. Flax fibers are separated from other parts of stems in the process called retting and are probably the first plant fibers used by man for textile purposes ( ). Nowadays flax cultivation is often limited because of its lower elastic property compared to cotton fibers. Thus the goal of this study was to increase the flax fiber quality using a transgenic approach. Expression of three bacterial genes coding for β‐ketothiolase ( phb A ), acetoacetyl‐CoA reductase ( phb B ), and PHB synthase ( phb C ) resulted in poly‐β‐hydroxybutyrate (PHB) accumulation in the plant stem. PHB is known as a biodegradable thermoplastic displaying chemical and physical properties similar to those of conventional plastics (i.e., polypropylene). The fibers isolated from transgenic flax plants cultivated in the field and synthesizing PHB were then studied for biomechanical properties. All measured parameters, strength, Youngapos;s modulus, and energy for failure of flax fibers, were significantly increased. Thus the substantial improvement in elastic properties of fibers from the transgenic line has been achieved. Since the acetyl CoA, substrate for PHB synthesis, is involved not only for energy production but also for synthesis of many cellular constituents, the goal of this study was also the analysis of those metabolites, which interfere with plant physiology and thus fiber quality. The analyzed plants showed that reduction in lignin, pectin, and hemicellulose levels resulted in increased retting efficiency. A significant increase in phenolic acids was also detected, and this was the reason for improved plant resistance to pathogen infection. However, a slight decrease in crop production was detected.