The influence of a weak magnetic field on the microstructure of the Zn electrode posits prepared in a thin concentric electrode cell is investigated. Mechanism o f the effect of the magnetohydrodynamic (MHD) flow on the morphological transiti on is proposed through experimental evidences. 1) In the early growth phase of a crystallite, the unbalanced force caused by the MHD flow on the crystallite, wh ich suspends in the electrolyte close to the previous branch, rotates its prefer ential growth orientation towards the downstream of the MHD flow; 2) A stable co ncentration profile is formed in the vicinity of the electrodeposits by the MHD flow. After the crystallite grows large enough to stick on its previous branch, and the concentration of the electrolyte is high enough for both the primary bra nch and its secondary branches, the single-crystalline dendrites will grow strai ght. But if the concentration of the electrolyte is small, the secondary branche s crowd to the side facing the upstream of the MHD flow and the primary branch i s forced to bend towards the downstream of the MHD flow. A spiral structure in t he branched electrodeposits grown in a magnetic field is formed by this mechanis m.