Abstract: Cubic boron arsenide (BAs) has attracted great attention due to its high thermal conductivity, however, its controllable, stable, and ideal preparation remains challenging. Herein, we investigated the effect of iodine-containing transport agents I₂ and boron triiodide (BI₃) on BAs synthesized and grown through chemical vapor transport. Results show that similar to the commonly used I₂, BI₃ accelerates the synthesis and improves the mass fraction of BAs from ~12% to over 90% at 820°C and 1.5 MPa, a value beyond the promoting effect of only increasing temperature and pressure. Both agents enhance the quality of BAs crystals by reducing the full width at half maximum by up to 10%–20%. I₂ agglomerates the grown crystals with twin defects (~50 nm wide), and BI₃ improves the crystal anisotropy and element uniformity of BAs crystals with narrow twins (~15 nm wide) and increases the stoichiometry ratio (~0.990) to almost 1. Owing to the boron interstitials from the excessive boron supply, the spacing of layers in 111 increases to 0.286 nm in the presence of I₂. Owing to its coordinated effect, BI₃ only slightly influences the layer spacing at 0.275 nm, which is close to the theoretical value of 0.276 nm. In the chemical vapor transport, the anisotropic crystals with flat surfaces exhibit single-crystal characteristics under the action of BI₃. Different from that of I₂, the coordinated effect of BI₃ can promote the efficient preparation of high-quality BAs crystal seeds and facilitate the advanced application of BAs.