A mutant of green fluorescent protein ($GFPmut3^*$) from the jellyfish Aequorea victoria was cyclized in vitro and in vivo by the use of a naturally split intein from the dnaE gene of Synechocystis species PCC6803 (Ssp). Cyclization of $GFPmut3^*$ was confirmed by amino acid sequencing and resulted in an increased electrophoretic mobility compared with the linear $GFPmut3^*$. The circular $GFPmut3^*$ was $5^{circ}C$ more thermostable than the linear form and significantly more resistant to proteolysis of exopeptidase. The circular $GFPmut3^*$ also displayed increased relative fluorescence intensity. In addition, chemical stability of $GFPmut3^*$ against GdnHCl revealed more stability of the circular form compared with the linear form.