Surveys show that the majority of Greek students use procedural strategies in regards to mental calculations with rational numbers, ie, they are just trying to apply rules without having the adequate understanding (Lemonidis, 2013, Lemonidis & Kaiafa in this issue). This study proposes a teaching intervention which aims to advance students understanding about mental calculations and enrich their conceptual strategies repertoire. For this purpose, three teachers of 6th grade were first trained and, later, asked to teach mental calculations operations with rational numbers for a period of three months. The post training understanding and skills of 66 students were compared with the answers of 462 5th and 6th grade students who participated in mathematics competition by using the same instrument (Lemonidis and Kaiafa in this issue). The results showed that students of the experimental group, exploited a larger pool of strategies and, especially, more conceptual strategies. It is interesting that teachers were not familiar enough with the strategies for executing mental operations with rational numbers. After the intervention, positive changes in their attitudes and knowledge were identified.