Onion is the one type of veggie that have long been cultivated by farmers. One multiplication of onion is using true shallot seed that having the good advantage than other. For this reason, it is important to conduct a research using castcing and regosol to cultivation onion. Regosol is weakly developed soils which is requires organic fertilizer with the right dose can improve the soil structure. The purpose of the research was aims to determine the right mixture of castcing and regosol for onion seed. This research was conducted in Green House of Universitas Muhammadiyah Yogyakarta from August 2017 to September 2017 using experimental research with single factor and consists of 4 treatment, castcing 20 % and regosol 80 %, castcing 30 % and regosol 70 %, castcing 40 % and regosol 60 %, castcing 50 % and regosol 50 %. Every treatment was replicated five times. The result of the research showed that castcing 50 % and regosol 50 % is the best treatment than the others. The results are percentage of germination 82 %, coefisient germination 50 %, height of the plant 26,17 cm, length of root 0,75 cm, total of leaves 3,4 blade, color of leaves score 4 from 5, fresh weight of plant 0,61 gram, dry weight of plant 0,04 gram, weight of fresh root 0,06 gram, weight of dry root 0,009 gram.