The Sb-flow-rate dependence of atomic ordering in Ga0.5In0.5P grown at about 650 degreesC on (0 0 1) GaAs substrates by metalorganic-vapor-phase epitaxy was studied. The Sb-flow-rate ratio f(R) (defined as [TDMASb]/([PH3] + [TDMASb])) was varied in a small Sb-flow-rate-ratio regime (f(R) less than or equal to 1%), where [TDMASb] and [PH3] indicate the Sb-source-gas (TDMASb: Tris-dimethy-lamino-antimony) flow-rate and phosphine flow-rate, respectively. CuPt-B-type atomic ordering appeared for 0% less than or equal to f(R) less than or equal to 0.015%. For f(R) approximate to 0.015%, a mixture of CuPt-B and TP (triple-period)-A ordering domains is formed, while for f(R) less than or equal to 0.05%, TP-A ordering was dominant. Based on these results, it was concluded that the surface structure changed from (2 x 1)to(2 x 3) in-between f(R) = 0.015% and 0.05%. The Sb-composition in solid ((1 - y)(Sb), where 1 - y is that in Ga0.5In0.5PySb1-y) tended to saturate for f(R) greater than or equal to 0.05%. It is noted that TP-A-type ordering appeared in the Sb-composition saturation regime; (1-y)(Sb) was 0.73% at f(R) approximate to 0.050% and, 1.6% at f(R) approximate to 1.0%, respectively. Thus, Sb incorporation appears to be influenced by the surface reconstruction.