Measurement factors for the dielectric properties of low dielectric loss materials (tan${dalta}{le}10^{-4}$) were investigated using the dielectric rod resonator method. It was shown that the relative conductivity (${sigma}_{r}$) should be controlled within a 5% to obtain the standard deviations of less than 0.07 for permittivity .epsilon.r and 0.06${ imes}10^{-4}$ for tan.delta.respectively. Surface resistivity (R$_s$) could be reduced when the surface roughness of parallelled conducting plate was less than 0.07 .mu.m. Measurement error for the permittivity was $pm$0.02% independent of probe loop size, whereas the error in Q value was reduced with the decrease in probe loop size and also with the increase in the absolute values of Q. Reliable Q values were determined with the probe loop size of less than 4mm. The accurate for the distance between the measuring probe loop and the sample could be obtained when the insertion loss of resonant frequency ranged -15dB - -30dB.