Pliers with adjustable jaw span have a first member (1) and a second member (2) which cooperate with each other. A long slot (13) is formed in the first member (1). The second member (2) is connected with the first member (1) by means of a pivot (3) passing through the long slot (13), and rotatable around the pivot (3) relative to the first member (1), while adjusting the span of the jaws by varying the position of the pivot (3) within the long slot (13). The edge of the long slot (13) is provided with ratchet teeth (14). The pivot (3) is engaged with the ratchet teeth (14) by a pawl (31), thus the pivot is movable within the long slot (13) in a first direction and immovable in a second direction which is opposite to the first direction. The pliers are convenient to operate because when the position of the pivot (3) is moved within the long slot (13), the user's operation is required only for the movement in one direction.