A pen base mouse is constructed to include a penholder holding a circuit board, the circuit board having a wireless transmitter for signal output to a computer, a cursor control unit mounted in one end of the penholder and adapted to control a pointer on a computer screen, and three button units including a left button unit, an intermediate button unit, and a right button unit, each button unit having a roller peripherally extended out of the penholder for rotating and pressing by the user to produce a respective control signal to the circuit board.