Two experiments conducted on laying hens, aged 50 and 52 weeks, were carried out to evaluate the effect of dietary Mg and its relationship to the Ca : NPP (non-phytate phosphorus) ratio using a various grain-size of limestone. The Ca : NPP ratio in each experiment was 12.8 and 18, respectively. Two Mg levels were evaluated in the first (1.56 and 4.0 g/kg) and the second experiment (1.52 and 4.0 g/kg). A fine-grained limestone (< 0.5 mm; FL) or a coarse-grained limestone (0.8–2 mm; CL) was used in the first experiment. In the second experiment, a mixture of both the aforementioned limestone forms (FCL) was used as the third alternative. The main parameters estimated in this study were egg production and egg shell breaking strength. In the first experiment, CL significantly increased hen-day egg production (P = 0.043) and Mg (in dietary concentration up to 4 g/kg) increased egg weight (P < 0.001). The addition of Mg to the mixed feed, together with CL, decreased yolk percentage (P = 0.008), increased egg shell percentage (P = 0.044), increased egg shell thickness (P = 0.014), and egg shell breaking strength (P = 0.003). Higher dietary Mg, together with a wider Ca : NPP ratio in the second experiment, increased egg production and egg weight (P < 0.001), but it did not influence egg shell breaking strength. CL increased egg shell breaking strength compared to the addition of FL, as well as FCL (P < 0.05), regardless of the Mg levels. Lower level of Mg with FL decreased ash content of shells (P = 0.004).