Thermal stress at an off-angled interface is considered as a model analysis for “weld line” in an injection-molded FRP. The off-angled interface is a line interface between two bonded anisotropic solids, at which the axis ofanisotropy in each solid is inclined to the interface. Applying the regular technique of integral transforms, we obtain the exact solution for temperature and stress induced by a spontaneous heat source. Especially for the temperature we get a closed-form solution in one term of the exponential function. Extensive numerical computations are carried out for a Nylon 66 compounded with 33 wt% glass fiber and show the stress field near the weld line (off-angled interface). The interfacial shear stress attains its maximum value more quickly than the normal stress. However, the highest normal stress on the interface increases with increasing off-angle, the highest shear stress has a maximum at 43-deg off-angle. The shear stress is 2.8 times greater than that in the isotropic solid and is also 1.34 times greater than that for 90-deg off-angle. This indicates that the lowest strength of the weld line would occur when the off-angle is about 43 deg for our case of Nylon 66.