Onion (Allium cepa L.) production in semi-arid area is primarily limited by fertilizer and water. In 2007/8 an experiment was conducted at Mekelle University, Northern Ethiopia to determine effect of N and P on yield and yield components of onion under different irrigation regimes. A split plot design was used. Two irrigation regimes (full and deficit) as main plot, factorial combination of four levels of N (0, 46, 92, 138 kg ha-1) and four levels of P (0, 20.1, 40.2, 60.3 kg ha-1) as a subplot with three replications were used. Statical Package Jmp 5.1 and M-STAT C was used as stastical...