All flannel/flannelette will fray, even the highest quality one. If you'd like to keep fraying to the minimum, put it in a mesh bag when pre-washing. Some people would zigzag the edges before pre-treating to keep fraying close to zero - for when you need every little bit and thread of your flannel piece.

Do not wash flannel or flannelette in hot water and do not put in the dryer.  If you wash your flannel/flannelette on the delicate cycle in warm or cold it water, it will drastically reduce pilling and shrinking.  Lay flat to dry or hang on the line.  If you absolutely need to put in the dryer use the lowest setting possible.