A controller assembly for a window blind, comprising: at least one transmission mechanism including a gear unit including a first major gear and a second major gear which are coaxially spaced apart from each other along a longitudinal axis, and an auxiliary gear set disposed to couple to both said first and second major gears (33, 33') such that when said gear unit is driven to operate, said first and second major gears (33, 33') rotate respectively in a first rotational direction and a second rotational direction which is opposite to the first rotational direction, and a transmission unit which is selectively coupled to rotate with a selected one of said first and second major gears (33, 33'); and a drive mechanism including a spool unit coupled to said at least one transmission mechanism, and a pulling cord coupled to and wound around said spool unit such that when said pulling cord is actuated to unwind from said spool unit, said gear unit of said least one transmission mechanism is driven to operate. at least one transmission mechanism including a gear unit including a first major gear and a second major gear which are coaxially spaced apart from each other along a longitudinal axis, and an auxiliary gear set disposed to couple to both said first and second major gears (33, 33') such that when said gear unit is driven to operate, said first and second major gears (33, 33') rotate respectively in a first rotational direction and a second rotational direction which is opposite to the first rotational direction, and a transmission unit which is selectively coupled to rotate with a selected one of said first and second major gears (33, 33'); and a gear unit including a first major gear and a second major gear which are coaxially spaced apart from each other along a longitudinal axis, and an auxiliary gear set disposed to couple to both said first and second major gears (33, 33') such that when said gear unit is driven to operate, said first and second major gears (33, 33') rotate respectively in a first rotational direction and a second rotational direction which is opposite to the first rotational direction, and a first major gear and a second major gear which are coaxially spaced apart from each other along a longitudinal axis, and an auxiliary gear set disposed to couple to both said first and second major gears (33, 33') such that when said gear unit is driven to operate, said first and second major gears (33, 33') rotate respectively in a first rotational direction and a second rotational direction which is opposite to the first rotational direction, and a transmission unit which is selectively coupled to rotate with a selected one of said first and second major gears (33, 33'); and a drive mechanism including a spool unit coupled to said at least one transmission mechanism, and a pulling cord coupled to and wound around said spool unit such that when said pulling cord is actuated to unwind from said spool unit, said gear unit of said least one transmission mechanism is driven to operate. a spool unit coupled to said at least one transmission mechanism, and a pulling cord coupled to and wound around said spool unit such that when said pulling cord is actuated to unwind from said spool unit, said gear unit of said least one transmission mechanism is driven to operate.