  We study the role of the guide field in relativistic magnetic reconnection in
a Harris current sheet of pair ($e^{\pm}$) plasmas, using linear theories and
particle-in-cell (PIC) simulations. Two-dimensional PIC simulations exhibit the
guide field dependence to the linear instabilities; the tearing or reconnection
modes are relatively insensitive, while the relativistic drift-kink instability
(RDKI), the fastest mode in a relativistic current sheet, is stabilized by the
guide field. Particle acceleration in the nonlinear stage is also investigated.
A three-dimensional PIC simulation demonstrates that the current sheet is
unstable to the RDKI, although small reconnection occurs in the deformed
current sheet. Another three-dimensional PIC simulation with a guide field
demonstrates a completely different scenario. Secondary magnetic reconnection
is triggered by nonlinear coupling of oblique instabilities, which we call the
relativistic drift-sausage tearing instability. Therefore, particle
acceleration by relativistic guide field reconnection occurs in
three-dimensional configuration. Based on the plasma theories, we discuss an
important role of the guide field: to enable non-thermal particle acceleration
by magnetic reconnection.
