Microinjection is one of the key technologies to introduce foreign molecules into the animal cell and embryos. Recently, a novel microinjection method for piercing relatively large egg (similar to phi 1 mm) utilizing the short electric pulse to pierce the biomembrane was developed. Although this method, electro-microinjection, was proved to be capable to pierce the stiff shell of fish egg without fatal damage, its mechanism has not been understood enough to optimize the parameter and the configuration of electro-microinjection. In this study, we have clarified the mechanism of electro-microinjection by numerically calculating the electric and thermal fields during electro-microinjection. A numerical model based on the time domain finite element method (TDFEM) with the measured reliable electric properties of fish egg was developed to calculate the intra-cell electric and thermal fields in detail. The calculation was performed in the conditions of minimal required pulse strength for piercing (227-1470 V) at various given pulse length (10 mu s-1 ms) that were obtained from our previous research. The results revealed that the applied pulse generates a locally focused high temperature near the needle electrode and an electric force of 80 mu N at its maximum which draws the fish egg to the injection needle. It was found that at least the local temperature of 330 K near the tip of piercing needle by Joule heating is required for electro-microinjection to decrease the rupture stress of the stiff biological shell of fish egg. (C) 2019 Published by Elsevier Ltd.