Provided are isolated polypeptides which are at least 80% homologous to SEQ ID NO: 474-643, 645-679, 681-755, 757-760, 4806-6390, 6395-6396, 6401-6895, 6897-7249, 7251-7685, 7687-7693, 7695-7700, 7702-7708, 7710-7796, 7798-7816, 7818, 7820-7837, 7839-7840, 7842-7861, 7863-8134, 8136-8163 or 8164, isolated polynucleotides which are at least 80% identical to SEQ I D NOs: 1 - 170, 172-267, 269- 424, 426-473, 761 -2486, 2489-2494, 2496-4803 or 4804, nucleic acid constructs comprising same, transgenic cells expressing same, transgenic plants expressing same and method of using same for increasing yield, harvest index, 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.
