Elastin-like polypeptides (ELPs) have a distinctive thermal property, transition temperature ($T_t$), which leads to phase transition. This thermal property depends on the molecular weight (MW) of ELP, ELP concentration, composition of the amino acids constituting ELPs, and ionic strength of the aqueous solution. In order to investigate the effects of ELP length, ionic strength and existence of fusion protein, ELP genes of three different sizes were cloned using the recursive directional ligation (RDL) method and expressed in Escherichia coli. Following purification, thermal behaviors of ELPs were monitored using a spectrophotometer with temperature scanning. The results of our study indicated that $T_t$ shifted to low in accordance with ELP length or increased ionic strength. Additionally, it was observed that $T_t$ was affected by the physical properties of the protein fused with ELPs.