Changes in environmental conditions such as temperature and humidity affect structural vibration properties. Other researchers have reported that modal frequencies varied significantly in a single day due to temperature change. Therefore, it is necessary to discriminate the variation of modal properties due to environmental change from those caused by structural damage, and quantify the environmental effect in vibration-based structural health monitoring and damage identification. Environmental conditions affect structural vibration properties in a complicated manner; this paper only investigates the variation of frequencies, mode shapes and damping with respect to temperature and humidity changes. A reinforced concrete slab, which was constructed and placed outside the laboratory, has been periodically vibration tested for nearly two years. This paper reports results obtained over that time for the first four modes. It is found that the frequencies have a strong negative correlation with temperature and humidity, damping ratios have a positive correlation, but no clear correlation of mode shapes with temperature and humidity change can be observed. Linear regression models between modal properties and environmental factors are built. A quantification analysis shows that variation of the elastic modulus of the material is the primary cause of the variation of modal properties.