A capacitorless one transistor (1T) semiconductor device whose data storage abilities are increased and leakage current is reduced is provided. The capacitor-less 1T semiconductor device includes a buried insulating layer formed on a substrate, an active region formed on the buried insulating layer and including a source region, a drain region and a floating body formed between the source region and the drain region, and a gate pattern formed on the floating body, wherein the floating body includes a main floating body having the same top surface height as one of the source region and the drain region, and a first upper floating body formed between the main floating body and the gate pattern.