A pull cord mechanism for a window shade includes a frame (1), a shade (2), a shade driving mechanism (3), and a bead chain or pull cord (4) for driving the shade driving mechanism (3). A sliding block (7) is slideably received in a barrel (5) receiving the pull cord (4). A handle (8) is mounted outside of the barrel (5) and connected to the sliding block (7). A pivotal plate (12a) is pivotably supported in a passageway (11) of the sliding block (7). A recess is defined in one of a rear side (12a1) of the pivotal plate (12a) and a wall (11a) of the passageway (11) facing the rear side (12a1). A positioning block (12c) is located between the rear side (12a1) of the pivotal plate (12a) and the wall (11a) of the passageway (11). A gear (12b) is located between the section (4a) of the pull cord (4) and a front side (12a2) of the pivotal plate (12a) opposite to the rear side (12a1).