In open pit mining drilling, like the other stages of mining, has a close relationship with the rock mass. The drilling process and its results are affected by various parameters of the rock material and rock mass. The effects of rock materials have been emphasized in various studies, however a lack of perfect knowledge of rock mass structural parameters may lead to unpredictable results. This paper presents a new classification system for specifying Rockmass Drillability index (RDi). For this purpose, six parameters of rock mass, including texture and grain size, hardness, uniaxial compressive strength (UCS), joint spacing, joint filling (aperture) and joint dip, have been investigated by physical modelling and rated. In this system, for each rock mass a rating from 7 to 100 is assigned. A higher rating represents a good-quality rock mass from the view point of drilling. Finally, considering the RDi rating, the drilling rate may be predicted at one of five modes; slow, slow-medium, medium, medium-fast and fast.