Digital elevation models (DEMs) of a coastal area in a time series are very useful to monitor annual and seasonal coastal morphologic changes. Topographic features in coastal regions change significantly within a relatively short period compared with landmass. However, it is difficult to obtain coherent interferometric SAR pairs over coastal areas. ERS-ENVISAT 30-minute tandem pair is useful to observe coastal regions. We studied an ERS-ENVISAT cross-interferometric pair with a perpendicular baseline of 1.4 km and a height ambiguity of 6 m. The small height ambiguity is favorable to construct a DEM in low relief coastal areas. Accurate coregistration was required because of the differences of the pulse repetition frequency and range sampling rate between the two sensors. Resampling of ENVISAT ASAR data was performed to have equivalent pixel spacing to the ERS SAR data. Range and azimuth offset were estimated to a sub-pixel accuracy using image intensity cross correlation. A larger window chip size than a normal case was used because it was difficult to distinguish typical features. Difference of 540 Hz Doppler centroid was estimated, and the cross-interferogram was highly coherent. As range bin increased, the difference of Doppler centroid also increased. The differences of Doppler centroid were about 450 Hz and 600 Hz, respectively in near and far range. It resulted in lower coherence in far range than in near range. Coherence was improved by applying azimuth and range common band filtering. Coherences over wetland in near and far range were about 0.8 and 0.5, respectively. ERS-ENVISAT cross-interferogram usually lost information in urban area. Accuracy of the DEM constructed by the ERS-ENVISAT 30-minute pair in a coastal area is to be evaluated.