In order to develop higher-voltage vacuum circuit breakers (VCB), the investigation of discharge phenomenon in vacuum is necessary. In vacuum interrupters (VI) with composite insulation systems, the discharge has complex and various patterns at the internal insulation failures. In this paper, based on the discharge development characteristics, we propose a discrimination method for the discharge pattern in vacuum. We measured the applied voltage, anode current, and shield potential waveforms, respectively. From the feature of the measured waveforms, we classified the discharge patterns. Furthermore, based on the relation between discharge development time and discharge path length, we estimated the discharge patterns and discharge path lengths. In addition, we conducted a verification experiment of the discrimination method using a VI model. Experimental results revealed that the estimated discharge patterns and discharge path lengths were consistent with the observed discharge ones and the effectiveness of the discrimination method was confirmed.