In order to study the effect of different sample diameters on the instability of eutectic structure, directional solidification experiment was carried out on Ni-Ni3Si alloy with sample diameters of 4 mm and 7 mm.The results show that the primary phase of coupled eutectic growth is Ni3Si phase and the instability of lamellar eutectic is more likely to occur on the sample with small diameter. The form of instability is a zigzag instability which leads to curved lamellar structure. The sample with smaller diameter gets higher thermal gradient, leading to the smaller undercooling of the alloy. Further, the instability of lamellar eutectic will happened at larger growth rates, and which can help to reduce the difference of diffusion growth of the eutectic phase, thus narrowing the width gaps between the two phases. In addition, this instability is related to the grain orientation.