We obtained the NMR spectrum and the spin-lattice relaxation time (T-1) for thin film samples by magnetic resonance force microscopy (MRFM). The samples were CaF2 thin films which were 50 nm and 150 nm thick. T-1 was measured at 18 K using a cyclic adiabatic inversion method at a fixed frequency. A comparison of the bulk and two thin films showed that T-1 becomes shorter as the film thickness decreases. To make the comparison as accurate as possible, all three samples were loaded onto different beams of a multi-cantilever array and measured in the same experimental environment.