The performance of diffusely reflecting boundary to improve the speech intelligibility in spherical spaces was quantitatively explored. The research was limited within geometrical acoustics. Omnidirectional point source was set at the sphere center, which is a typical position to cause sound focusing easily in a sphere. The boundary sound absorption coefficient was uniformly distributed. By calculating the sound impulse responses and related parameters, comparisons were made between the sound fields under two boundary reflecting patterns in spheres. Analytical and acoustical radiosity methods were used for the cases of the two types of boundary respectively. Clarity (C50) was adopted as the objective parameter to assess the speech intelligibility. The results showed that C50 is low with specularly reflecting boundary, and diffusely reflecting boundary generally enhances C50, especially in the sound focusing area near the sphere center. However, the improvement caused by the boundary reflection pattern may be still inferior to that caused by room shape.