The objective of this study was to xplore the spatio-temporal variability of soil organic matter (SOM) in the urban-rural transition zone of Beijing. SOM content in agricultural soils were measured in 1980, 1990 and 2000 in Daxing County of Beijing in-situ and data of 1980 and 1990 were obtained from the National Soil Survey (NSS). Descriptive statistics and geostatistics were used to analyze the data and the kriging method was applied to map the spatial patterns of SOM. The results showed that mean SOM was 9.95g kg−1 in 1980, 12.76g kg−1 in 1990 and 12.89gkg−1 in 2000. SOM was spatially correlated at a larger distance of 32.0km in the E–W direction for the three years, and at a shorter distance of 24.6, 23.3 and 19.0km in the N-S direction in 1980, 1990 and 2000, respectively, which showed that there was more variability in SOM in the N-S areas across the period of 20years. The mapping showed a decreasing trend of SOM from north to south across the county. SOM levels were classified into six levels (b6, 6-10, 10-12, 12-15, 15-20 and N20g kg−1) based on the standards set by the NSS. The SOM slightly increased from low to high levels from 1980 to 2000. The main factors affecting SOM levels were the soil texture, land use and farming practices. The increasing trend might be attributed to the widespread practices of mulching and organic manure applications.