A summary is given of the error analysis of the National Bureau of Standards (NBS) 1016-mm guarded hot plate (GHP) apparatus, with emphasis given to new ideas or techniques. The report discusses the following items: a brief introduction to the principle of measurement, a rationale for the method of summing of uncertainties, a list of individual uncertainties, a description of the innovative plate temperature control method, and a derivation of the error due to systematic uncertainty for the heat flow meter apparatus.