Time domain reflectometry has been used extensively for measuring soil water content (θv) based on the relationship between θv and dielectric constant (K). The objective of this study was to develop an improved θv-K equation by including soil bulk density (ρb). Laboratory-measured θv, ρb, and K data on 12 soils of different textures were used to establish the calibration equation. The performance of the new equation was then evaluated on independent soils under field conditions. The results showed that the Topp equation (Topp, G. C., J. L. Davis, and A. P. Annan. 1980. Electromagnetic determination of soil water content: Measurements in coaxial transmission lines. Water Resour. Res. 16:574-582) worked well on soils of different types and textures, in the intermediate ρb range (1.20-1.40 g cm−3) but produced large discrepancy on soil with low ρb values. On the other hand, the errors of θv estimated from the new equation were generally within ±0.02 cm3 cm−3. In addition, the new equation performed better than the previous calibration equations (Roth, K., R. Shulin, H. Flühler, and W. Attinger. 1990. Calibration of time domain reflectometry for water content measurement using a composite dielectric approach. Water Resour. Res. 26:2267-2273; Malicki, M. A., R. Plagge, and C. H. Roth. 1996. Improving the calibration of dielectric TDR soil moisture determination taking into account the solid soil. Eur. J. Soil Sci. 47:357-366.) that had considered soil bulk density.