We had developed a high-quality flexible coaxial cable with a diameter of less than 1 mm. It is difficult to carry out precise measurement using the conventional coaxial connectors that are commercially available because this small-diameter coaxial cable has a specific structure. Therefore, we have proposed a precise high frequency measurement method for determining the characteristics of a small-diameter coaxial cable without expensive connectors. In this method, a time domain reflectometry (TDR) measurement system and a microwave vector network analyzer (VNA) are used with a microwave probe station. The cables are molded using resin and cut vertically with a cutting tool. The surfaces of the cable edges are polished to obtain a good electrical contact. In this measurement method, a 40 GHz microwave contact probe was used on the microwave probe station. We have experimentally demonstrated the precise measurement of the characteristics of 0.8-mm-diameter coaxial cables. The impedance distribution and S-parameters were successfully evaluated by TDR and VNA measurement respectively. By introducing the new measurement method, we obtained high quality characteristics of the especial small diameter coaxial cable by 40 GHz. A very uniform impedance profile of 50 C2 was obtained by the TDR measurement. S-parameter (S21) of -15 dB/m at 40 GHz was measured by a vector impedance measurement using the VNA. The impedance distortion at the probe contact was negligible.