The fuzzy approach has proved its ability to describe complicated systems in many cases where conventional mathematical methods have failed. Applications can be found in many branches of science including chemical engineering. The cost estimation of chemical plants is commonly referred to as being more of an art than a science. This reflects the fact that cost estimation problems can only partly be solved by analytical or statistical methods. It is for this reason that we propose the fuzzy approach. In this paper the fuzzy concept is applied to cost estimation by developing a fuzzy model for the Lang factor. Some general considerations of the fuzzy approach to cost engineering are also presented.