Provided are isolated polypeptides, 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, early flowering, grain filling period, harvest index, plant height, and/or nitrogen use efficiency of a plant.