It is investigated in the present paper how an initially nonspherical particle in a shrinking core regime changes its shape temporally, and how much the eccentricity of the particle affects the total reaction rate (i.e., conversion), The pseudo-steady state assumption may be applied because the core shrinking rate is sufficiently low. The resultant Laplace equation is solved by using the boundary element method to yield the reactant concentrations on the core, which are then used to predict the shrinking process of the core, The shape change of a spheroidal core is also analytically predicted with asymptotic analysis. It is shown that the two different approaches produce equal results. The spheroidal core becomes more and more eccentric with time, to be very slender when the core size is much reduced. The more nonspherical the particle, the faster the total reaction rate. For a highly nonspherical particle with an aspect ratio of 3.2, one may undergo about 17% error in determining the reaction constant if the spherical core is employed in the data analysis.