The mass absorption efficiency (MAE) of black carbon (BC) was measured for the ambient aerosols in urban and suburban areas and exhaust plumes from motor vehicles from September 2010 to September 2011 in Xiamen, China. Ambient aerosols, collected on quartz fiber filters, were analyzed for BC loadings using an OC/EC Carbon Aerosol Analyzer and measured for optical attenuations with a laser/LED transmissometer. The MAE of BC in Xiamen shows a large temporal and spatial variability, varying from 0.47 to 5.15 m2/g. In the same month (May 2011), the average MAE of BC in suburban area (0.50 m2/g) was one third that in urban area (1.50 m2/g), suggesting that the variance of MAE was related to the specific emission sources of BC. Here we report the large variation in MAE of BC measured during different sampling times and for different sources in Xiamen. The average MAE was 1.69 m2/g and the coefficient of variance was 52%. Factors contributing to the variability were also discussed, as well as the uncertainties caused by different carbon analysis methods, working wavelengths for optical attenuation measurements, and relative humidity when measuring the optical attenuation of the aerosol filters. The results suggest that if a constant value of MAE within the range of 0.47–5.15 m2/g is used to quantify the BC concentration, then this would bias the results with a maximum factor of 11. Site-specific values of MAE are recommended for long-term stationary monitoring of ambient BC in air quality monitoring stations.