Photometric standard stars for L' and M filter bands

L' (3.8 microns) and M (4.9 microns) photometry of 20 primary and 21 secondary standard stars that are sufficiently faint for large telescope observation is presented. The estimated uncertainties in the magnitudes of the primary standards are 0.01 at L', 0.02 at M, and 0.015 for the L'-M color. A color transformation between the present L' magnitudes and the L magnitudes of Elias et al. (1982) is obtained and found to be near agreement with that predicted for black-bodies.