Y3+ substituted mullite Y(x)Al(6−x)Si2O13 nanoparticles where x varied from 0.005 to 0.05 have been synthesized via co-precipitation technique. X-ray diffraction results revealed that orthorhombic mullite was the major phase in the samples of x = 0.0–0.025, whereas corundum α-Al2O3 was predominant at high Y3+-ion content of x = 0.05. Transmission electron microscope images showed orthorhombic-like structure for the pure sample. Meanwhile, the doped samples exhibited similar morphologies of larger particle sizes associated with small amount of glassy liquid phase. FT-IR spectrum evinced the formation of corundum particularly at high Y3+ ion content (5 %). The photoluminescence emission spectra were strongly affected by the Y3+ ion content. Moreover, mullite sample doped with 0.5 % Y3+ ion achieved the minimum electrical resistivity of 0.28 × 109 Ω cm and the minimum dielectric loss value of 0.37 in the radiowave frequency region (10 MHz) as well as the minimum dielectric loss value of 0.41 in the microwave frequency region (1 GHz).