The variation of critical distances under the influence of different reflection conditions is investigated using one-speed neutron transport calculations for slabs and spheres. The spherical harmonics method is Used for the solution of criticality, problems. Various order P-N calculations are performed considering linearly, anisotropic media. The Marshak boundary, condition is used in a modified form to take into account an infinite reflector case. The critical equations are derived for both slabs and spheres. The critical half-thicknesses and critical radii are determined depending on the variables, reflection coefficient and anisotropy parameter for a given critical eigenvalue. It is seen that critical thicknesses of a slab decrease regularly approaching zero value as the reflection coefficient approaches to unity. However, critical radius of a sphere follows a different trend decreasing to some constant value before the reflection coefficient reaches the unity. Then no more decrease is observed by, further increase of the reflection coefficient. Numericat results and comparison with the literature are given.