Abstract: Electrical resistivity and initial cracking time of composite cement mortar materials mixed with fly ash were measured using an non- contacting resistivity meter and ellipse - ring cracking apparatus with an automatic monitoring system respectively. The result shows there are close relation ships between early resistivity change and hydration process as well as cracking sensitivity of composite cement mortar materials mixed with and without fly ash. With the increase of fly ash(FA) content, cracking sensitivity of fly ash mortar is decreased due to the delay of feature point of hardened structure in resisitivity curve. However, the limited fly ash content is favorable for other properties of composite cement mortar materials mixed with fly ash.