The present invention relates to an electroless nickel plating solution composition, a flexible printed circuit board and a manufacturing method thereof, and more particularly, to an electroless nickel plating solution composition, a flexible printed circuit board and a manufacturing method thereof capable of simultaneously satisfying plating characteristics respectively required for a pad unit and external connection units of the flexible printed circuit board by forming a nickel plating layer having a vertical growth structure with the electroless nickel plating solution composition including a water-soluble nickel compound, a reducing agent, a complexing agent and a vertical growth inducer.