Provided are isolated polypeptides which are at least 80% homologous to SEQ ID NOs: 182-184, 186-202, 204-216, 219-223, 225, 227-232, 235-236, 238, 240-260, 262-268, 270-275, 277-287, 289-297, 3651-3671, 3686, 3720-3721, 3724, 3727, 3735, 3754, 3774, 3795-4304, 4316, 4374, 4425, 4464, 4481-4813, 4824, 4833, 4843-4844, 4867-4869, 4888, 4890-4891, 5005-5050, 5053-5070, 5093, 5217, 5231, 5233, 5239, 5246, 5255, 5257-5296, 5412, 5415-5429, 5447-5456, 5465-5673, 5675-5686, 5688-5695, 5697-5698, 5700, 5702-5707, 5709-5715, 5717-5785, 5831, 5869, 5980, 6010-6043, 6045-6053, 6055-6093, 6132, 6383, 6405, 6493, 6523, 6533-6537, and 6563-6589, isolated polynucleotides encoding same, 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.