This paper presents error analysis of a simple method for verifying the phase of transmission measurements — for example, as made using instruments such as Vector Network Analyzers (VNAs). Phase verification method by using air line as a phase standard has been published by N. Ridler and M. Horibe. Examples are given that show the method being used at millimeter wavelengths (i.e., at frequencies up to 33 GHz) in the 3.5 mm coaxial line size. It is trusted that the method will be suitable for use by calibration laboratories seeking verified and/or accredited measurements in VNAs. An air dielectric coaxial line (air line) is commonly used as the reference impedance and absolute phase standard in a VNA measurement. In the method, phase measurement accuracy is verified by comparing between dimensional length and electrical length, analyzed by measured value of transmission phase, of air line. However, reflection characteristics of air line at connection interfaces affects to error of analytical results of electrical length.