Buletin Keairan media Informasi Kegiatan Penelitian Keairan, Vol. 2 No. 2, Desember 2009, ABSTRAK : In most cases, old dams like for instance the Krenceng Dam which was completed in the year 1977, are showing the problem of weak layer in the interface between spillway wall and embankment. This may lead to a problem in dam safety if not treated properly. The geotechnical investigation carried out in the year 2000 indicated that the NSPT was less than 5 blows/30 cm depth, reflecting weak soil in the area adjacent to the interface. To assure safety of the dam, the Ground Penetrating Radar (GPR) or Geo-radar was applied to study the dam body condition. Investigation results by using Geo-radar, show in general that the dam body is still in good condition, except for the interface. The weak layer at 6,0 m depth may potentially to be sliding down. Whereas from the stability analysis results at interface it was revealed that the safety factor (SF) without earthaquake was close to the minimum reguired SF, and SF with earthquake was less than one. Conunter-measures byusing stone counerweight at the downstream was recommend for increasing the safety factor of slope stablility. It was also recommended that the retaining wall of spillway should ber provided with some wheepholes to reduce pore pressures.