The coalescing mechanism of two in-line oblate-cusped bubbles rising in a non-Newtonian fluid has been qualitatively and quantitatively studied by using particle image analyzer. Through effects of the shear thinning and viscoelasticity on the flow structure, the acceleration of the trailing bubble to the leading bubble is evident. While two bubbles are getting closer, the drained liquid is circulated to the back of the trailing bubble. To encounter these dynamic flow structures, the bubble shape evolution for the leading bubble in each stage is from oblate-cusped to elongated oblate-cusped, to invert pentagonal, and to oblate. Suppressed by the trailing bubble and the flow structure, the shape of leading bubble changes from oblate-cusped to oblate, to flattened oblate, and to flattened oblate with concave surface.