GCaMP is one of the most widely used calcium indicators in neuronal imaging and calcium cell biology. The newly developed GCaMP6 shows superior brightness and ultrasensitivity to calcium concentration change. In this study, we determined crystal structures of Ca 2+ -bound GCaMP6 monomer and dimer and presented detailed structural analyses in comparison with its parent version GCaMP5G. Our analyses reveal the structural basis for the outperformance of this newly developed Ca 2+ indicator. Three substitution mutations and the resulting changes of local structure and interaction explain the ultrasensitivity and increased fluorescence intensity common to all three versions of GCaMP6. Each particular substitution in the three GCaMP6 is also structurally consistent with their differential sensitivity and intensity, maximizing the potential of using GCaMP6 in solving diverse problems in neuronal research and calcium signaling. Our studies shall also be beneficial to further structure-guided optimization of GCaMP and facilitate the design of novel calcium indicators. GCaMP6, calcium indicator, structural basis, ultrasensitivity