$CaNb_2O_6:RE^{3+}$ (RE=Sm or Eu) phosphor powders were prepared with different contents of activator ions by using solid-state reaction method. The X-ray diffraction patterns exhibited that the phosphors synthesized with different activator ions showed an orthorhombic system with a main (131) diffraction peak. The maximum size of the grain particles, determined from the measurement of scanning electron microscopy, was observed at 0.05 mol of $Eu^{3+}$ ions and at 0.01 mol of $Sm^{3+}$. As for the $Eu^{3+}$-doped phosphor powders, the excitation spectra were composed of a broad band peaked at 278 nm and several weak bands in the range of 350~500 nm, and the highest red emission spectrum was observed at 0.15 mol of $Eu^{3+}$ ions. As for the $Sm^{3+}$-activated phosphor powders, three strong emission bands under excitation at 273 nm were observed at 570, 612, and 659 nm, respectively. The intensities of all the emission bands approached maxima for 0.05 mol of $Sm^{3+}$ ions. The optical properties show that the $Eu^{3+}$- or $Sm^{3+}$-doped $CaNb_2O_6$ powders are promising red-orange emitting phosphor powders applicable to full-color photonic devices.