Abstract Magnetic and superconducting properties of (Gd,R) 1.85Ce 0.15CuO 4 with R = Pr and Sm have been studied. Antiferromagnetic ordering of Cu is observed in Gd-rich samples, and superconductivity appears in R-rich samples. With increasing R content, antiferromagnetism - superconductivity transition occurs. The temperature - R concentration phase diagram for (Gd,R) 1.85Ce 0.15CuO 4 is reported. Resistivity measurements reveal that the substitution of R for Gd results in an increase of effective charge carrier number. The antiferromagnetic ordering of Gd and Sm coexist with superconductivity at low temperature. The magnetic behavior of Gd and R ions varies due to the interaction between Gd and R ions. A possible explanation for the effect of R substitution is discussed.