This paper describes the investigation of concrete cracks of 2 existing reinforced concrete structure buildings with each three stories, which were constructed on a soft stratum in 1965 and 1972 respectively. In this study, the self-strain stresses due to the differential settlement, concrete drying shrinkage and thermal stress were evaluated with the related concrete crack width. Also, the concrete crack behaviors of the structural member due to variation of air temperature and relative humidity were studied. The following findings were obtained from this study:
(1) For the concrete crack width due to self -strain stress in exiting buildings、 maximum crack width to average value of crack width was scattered within the limit of 4 times.
(2) Daily concrete crack width behavior mainly depends on air temperature with or without time lag.. But yearly concrete crack width behavior does not always depend on air temperature. Because, the structural behaviors of each member are influenced by differential restraint effect and movement of the building foundation due to thermal stress.