Urbanization has  already significantly influenced urban thermal environment. Two monitoring stations were built both in urban and suburban Nanjing. The spatio-temporal characteristics of soil moisture, starting from June 1st, 2009 to June 7th, 2010, under three different land covers (bare soil, grass, concrete) within 1 m depth were studied. The result indicated that soil moisture in urban area was lower than that in suburban area, and revealed an obvious “urban dry island” effect in soil. The corresponding annual average urban dry island intensity (UDII) was approximately -7.4%. The UDII reached its gully and peak value in January and July, respectively, and the monthly average UDII was about -2.1% and -20.5%. In addition, soil moisture in suburban area increased with depth but this tendency is not significant in urban area. Moreover, it was observed that soil moisture under different land covers presented different characteristics. In urban area, daily soil moisture under bare soil cover was more variable than that under grass and concrete covers. The variation ranges of soil moisture under the three covers were all great. The sequences of yearly average soil moisture under the three covers were grass(19.0%)<concrete(26.4%)<bare soil (29.5%) in urban area and concrete(27.4%)<grass(34.7%)<bare soil (36.2%) in suburban area. The fluctuation of soil moisture in urban area is less severe than that in suburban area due to the large area of artificial land covers. The causes of the diversities in soil moisture were discussed and the urban heat island effect is one of the main reasons induce urban dry island effect.