Provided are isolated polypeptides which are at least 80% homologous to SEQ ID NOs: 552-897 and 6029-10629, isolated polynucleotides which are at least 80% identical to SEQ ID NOs: 1-551 and 898-6028, nucleic acid constructs comprising same, transgenic cells expressing same, transgenic plants expressing same and method of using same for increasing yield, abiotic stress tolerance, growth rate, biomass, vigor, oil content, photosynthetic capacity, seed yield, fiber yield, fiber quality, fiber length, and/or nitrogen use efficiency of a plant.