Today the effects of grouting are usually confirmed by the results of permeability tests but this method is not enough to show the changes in mechanical properties of rock masses. Although many investigators use the in situ tests for evaluation of rock mass mechanical property improvement. But this tests are time consuming and expensive. Grouting reduces the permeability and improves the condition of joints and ultimately increases the rate of rock mass classification in rock engineering. So with measurement of rock mass quality index values (Q-value) in cores obtained from grouted boreholes, the efficiency and success in improving the mechanical properties of rock mass can be showed. This paper for first time introduces Q-logging as a simple method to assess the impact of grouting in improvement of the rock mass quality. Here in, the results of Q-Logging in trial injection panels in the Bakhtiary, Bazoft and Khersan II dams have been examined. The deformation modulus were calculated from the Q-Logging for before and after of grouting. Results show that there is a good agreement between calculated rock mass parameters based on the Q-Logging method and the measured from in-situ test in the studied site. This agreement confirms the efficiency and applicability of the Q- Logging method for assessment of grouting success as well as the estimation of the rock mass parameters in grouted areas. Also it has been shown that the deformation modulus in weak rock mass with low quality has been more improved than rock mass with beater quality.