In this article, influence of diatomite as a natural adsorbent in immobilization of radioactive borate liquid waste in the cement-diatomite matrix was investigated. Cylindrical concrete mixtures were prepared with different amounts of diatomite additives at three different w/c values. Mechanical strength tests were carried out to determine the uniaxial compressive strength of the samples. A new dynamic column type leaching tests were applied to the solidified waste forms, and leach rates of Cs-137 and Co-60 were determined according to different diatomite additives. The experimental results indicated that the increasing of the diatomite additive amounts resulted high isolation performance of the solidified waste form. However, the results revealed that mechanical strength decreases with increasing borate diatomite amount. Results show that 9% amount of natural diatomite is effective as an admixture of the cementation process for achieving long-term stability of solidified borate waste. Optimum strength and leach values were determined as 11.5MPa with leaching rates of 7.5x10(-4) and 3.2x10(-5)cm/d for Cs-137 and Co-60, respectively.