Tetragonal K2NiF4-type Ca-2(Mn1-xNbx)O-4 with the space group 14(1)/acd was synthesized in the range 0.0 less than or equal to x less than or equal to 0.15. Ca-2(Mn1-xNbx)O-4 is an n-type semiconductor and exhibits the hopping conductivity in a small polaron model. The number of carriers (n) is approximate to6.5 X 10(26) m(-3) (x = 0.0) and 2.3-2.5 X 10(27) m(-3) (x = 0.05-0.15). At 320 K, the hopping mobility (mu (h)) is approximate to2.2 X 10(-9) m(2).V-1 s(-1) (X = 0.0) and 1.0-1.2 x 10(-8) m(2).V-1.s(-1) (x = 0.05-0.15). The electrical conductivity is influenced by the Mn3+ ion content and the (Mn, Nb)-O distance.