The Process Plants of Qatar Fertilizer Company (QAFCO) are located at 2 sites in Mesaieed Industrial City. The new site for QAFCO 5 and 6 is located at about 3 km west of the old site of QAFCO 1 to 4 Plants. Q5 and Q6 Projects were the biggest expansion activity ever undertaken by QAFCO, increasing the ammonia and urea capacities by 1.6 and 2.8 million Metric Tons (MT) per year, respectively. The En-gineering, Procurement, Construction and Commissioning (EPCC) contract for this project was signed with a consortium comprising of Saipem of Italy and Hyundai of South Korea in 2 stages. The Q5 contract was signed on the 2nd of December 2007. Q6 Contract was signed as an amendment to Q5 Contract on 9th of October 2009. This paper describes QAFCO’s experience in the commissioning and start-up of the process facilities, the problems encountered and key lessons learnt during the commissioning.