Alternative battery technologies are required to meet growing energy demands and to solve the limitations of the present energy technologies. As such, it is necessary to look beyond lithium-ion batteries. Zinc batteries enable high power density while being sourced from abundant and cost-effective materials. In this paper, the effect of the applied current and electrolyte flow rate on the early stage of Zn dendrite formation was characterized by in situ electrochemical liquid phase transmission electron microscopy (EC-LPTEM). For the first time, the square root relation is revealed between time and Zn dendrite growth on the lateral direction, indicating a diffusion-limited growth. It is intriguing that a higher applied current leads to longer incubation time. In situ EC-LPTEM can provide a useful strategy for understanding characteristics of unstable dendritic growth. The finding can help rationalize the electrode engineering design and parameters selection to avoid dendrite formation.