The internal stress in crystalline thermoplastics, polyphenylene sulphide (PPS), reinforced by carbon fibers of 30 mass% was measured by the diffraction method using synchrotron with energy of 12.3 keV. The stress in the matrix was determined by the sin²ψ method with side-inclination optics of transmitted X-ray diffractions. Using skin-layer strips cut parallel, perpendicular and 45° to the molding direction of the injection molded plates, the matrix stress was measured under the uniaxial applied stress. The matrix stresses in the fiber direction, σ₁^m and perpendicular to the fiber direction, σ₂^m, and shear stress τ₁₂^m were expressed as the functions of the applied (macro) stresses, σ₁^A , σ₂^A , τ₁₂^A as follows: σ₁^m = α₁₁σ₁^A + α₁₂σ₂^A , σ₂^m = α₂₁σ₁^A + α₂₂σ₂^A , ₁₂^m = α₅₅τ₁₂^A and the stress-partitioning coefficients, α₁₁, α₁₂, α₂₁, α₂₂, α₅₅, were determined. The coefficients determined by the transmission method are fairly close to the reported values determined by the reflection method. The experimental values were at least qualitatively agreed with the prediction derived based on micromechanics. The quantitative difference between experiment and prediction is mainly due to the neglect of the distribution of fiber orientations in the micromechanics prediction. Tensile residual stresses were measured in the matrix in the transmission method and were larger in the fiber direction than in the parallel direction. These residual stresses were caused by the mismatch of the thermal expansion coefficient between matrix and fibers.