Shapes and rising velocities of single bubbles rising through vertical square ducts were measured under stagnant, laminar and turbulent flow conditions to examine the applicability of available empirical correlations for singe bubbles in vertical pipes to those in square ducts. Experiments were carried out by using three different ducts, the widths W of which were 13.0, 16.2 and 27.2mm. The ratio λ of sphere-volume equivalent bubble diameter d to W was varied from 0.2 to 2.0. As a result, it was confirmed that (1) empirical correlations for evaluating terminal velocities of a bubble in a pipe are applicable to a bubble in a square duct, provided that the characteristics length of duct is evaluated as (4A/π) ^0.5 where A is the flow area, and (2) bubble shapes in square ducts are well expressed as a function of the liquid Reynolds number and the Eötvös number.