Polymer fibers were used in structural applications as micro reinforcement in composite materials having concrete matrix. Comparing to steel, glass and carbon fibers, polymer fibers behave visco-elastically or visco-elasto-plastically. Such fibers having moderate elastic modulus are characterized by relatively large elastic deformations and pronounced Poisson’s effect during stretching. Concrete prisms with dimensions 10x10x40 cm were fabricated, having different amounts of 3 cm long and 0.75 mm in diameter polymer fibers. All prisms were tested under four point bending conditions. Elaborated numerical model for crack opening in fiberconcrete was exploited for prism load –deflection mechanical behavior prediction under four point bending conditions. Experimental pull-out results were successfully used as an input data for the model.