In the iron smelting process, slag, which is production waste, usually forms the majority of remains found at ancient metallurgical sites. In Ban Kruat, Buriram Province, Northeast Thailand, at least 50 slag-bearing sites have so far been documented, which can be divided into nine clusters. One of the clusters, Ban Sai Tho 7 group, demonstrates a quite unique spatial organisation of slag mounds. Here, 10–11 mounds of circular or ellipsoid shape were found surrounding a flat area of 350 × 400 m. Two different stages of iron production were identified between these two areas: iron smelting at the circular mounds and iron smithing on the central plain. Compared with the nearby neighboring heap, this is a comparatively large concentration of slag, and possibly illustrates the long dynamic history of iron-smelting production. Although in 2009 and 2010, this site was estimated to have existed in the Iron age by accelerator mass spectrometric (AMS) dating of in-slag charcoal, the result only reveals the early production of iron-smelting regarding the law of superposition. Therefore, in order to assess the duration of production, optically stimulated luminescence (OSL) dating was employed to date technical ceramics (i.e., furnaces and tuyères) from the topmost layers of the surface slag heap in order to indicate the late or terminal iron-smelting production. The dates of the technical ceramics that came from the different sides of the slag heap show two different periods: the early 10th to the early 11th centuries (around 1000–1100 years ago) and the early 17th century (around 370 years ago). More significantly, the old samples were at the edge of the southern part, while the young samples came from the topmost of the northeastern slag heap. Therefore, these two furnaces were possibly used in different periods and distributed in different areas of the slag heap. The result also illustrates the terminal period (the early 17th century) of metallurgy in the ancient Angkor highlands, particularly at the Ban Sai Tho 7 site. An iron-smelting activity at Ban Sai Tho 7 seems to exist in the same location for several centuries. Moreover, OSL dating is an alternative method to directly date the late period of iron-smelting production.