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The porcelain ceramic insulators have been used for outdoor insulation applications for a long time due to their impressive electrical and mechanical properties. The lightning impulse withstand voltage and flashover voltage characteristics of the insulators are important for insulation coordination of transmission lines and substations. This study presents a computer-based simulation methodology to determine the positive polarity lighting impulse flashover voltage of porcelain suspension and station post-insulators. The flashover voltage is determined by evaluating the streamer inception, propagation and sustenance criteria along the suitable paths based on finite-element-method-based electric field computation. The streamer propagation path is found through air along the dry arcing distance rather than along the creepage path of the insulator. The lightning impulse flashover voltage of suspension and post-insulators are computed based on simulation. The comparison of computed results with the experimental results gives a good agreement between them.