This paper describes an improved microwave digestion procedure for steel samples of various kinds intended for inductively coupled plasma atomic emission spectrometry, including low alloy steel, stainless steel, alloyed tool steel, and high-speed steel. Their resulting solutions were used for the simultaneous quantification of alloyed elements together with several light elements of boron, aluminum, silicon, and phosphorous. Conventional digestion methods, using materials such as hydrochloric and nitric acids including aqua regia, and sulfuric and phosphoric acids with a fuming treatment, showed poor ability to decompose certain steels completely or to be simultaneously quantified with some elements. By contrast, microwave digestion with an acid mixture of hydrochloric acid, hydrofluoric acid, nitric acid, sulfuric acid, and distilled water with a 2:1:1:1:5 volume ratio fully decomposed various steel samples. For inductively coupled plasma atomic emission spectrometry, the suggested procedure is applicable to quantify boron, aluminum, silicon, phosphorous, titanium, vanadium, manganese, cobalt, copper, molybdenum, and tungsten.