Understanding dynamic mechanical response of rocks subjected to extreme loadings is necessary in rock engineering analysis and design. The dynamic increase factor (DIF), is usually defined as the ratio of the dynamic strength to the quasi-static strength in uniaxial compression or tension, which has been widely accepted as an important parameter to measure rate sensitivity of brittle materials. This paper summarizes the features of rate dependency of rocks and systematically reviews the expressions for the relationship between strain rates and mechanical strength of geo-materials. In addition, based on a number of experimental results, an aggregated DIF for compressive and tensile strength of rocks is proposed, which can provide a good prediction of rock strength over a wide range of strain rates.