The use of lightweight cemented clay has wide applications in infrastructure rehabilitation and in the construction of new facilities. Effects of water content, cement content, air content and fly ash (FA) on the unit weight and strength of lightweight cemented clay are presented and analysed. FA decreases the liquid limit and the plasticity index of the lightweight cemented clay. Consequently, it improves the unit weight, strength and workability of the lightweight cemented clay. For the same generalized stress state, w/wL (ratio of water content to the liquid limit), lightweight cemented clay with a higher replacement ratio has a higher strength for the same input of cement owing to a higher cement/clay-water ratio, C/w. The FA fixation point is a practical indicator to obtain a minimum FA replacement because it is simply obtained from an index test. The prediction of unit weight of the lightweight cemented clay for different water contents, air contents, FA replacement ratios and cement contents is performed and verified. From the critical analysis of test results, a mix design method to attain the target strength and unit weight is suggested. This method is useful from both engineering and economic viewpoints.