A FIELD experiment was carried out on the farm of Faculty of Agriculture, Mansoura University, Mansoura, Egypt during the winter season of 2016 to study the effect of bio-fertilization under different level of nitrogen and potassium (75% and 50% from the full dose) on the growth and yield of Onion plants. The obtained results showed that the microbial inoculation leads to a significant increase in growth parameters (foliage heights, number of leaves and dry weights). As well as NPK-contents as responded to all treatments under investigation in all stages of plant growth. The same trend was observed in yield parameters (bulb weights, total bulbs yield, Total soluble solids%, Dry matter% and NPK contents in bulb tissues). The treatment T4 (A. chroococcum + 75 % dose of NK) gave the highest total yield with an increase by 6.19% over the control. Also, the bio-fertilization has a pronounced increase in microbial count in comparison with the mineral fertilization. The obtained results of PGPR effects on growth parameters and yield of onion recommend their use as an alternative tool to reduce chemical fertilizers.