Significance Myosin-5a is a molecular motor that functions as a cargo transporter in cells. The motor function of myosin-5a is regulated by calcium via the calmodulin bound to the first isoleucine-glutamine (IQ) motif (IQ1) of myosin-5a. Here, we solve the crystal structure of a truncated myosin-5a containing the motor domain and the IQ1 complexed with calcium-bound calmodulin. Comparison of the structures of the IQ1 complexed with calmodulin with or without bound calcium reveals the calcium-induced conformational changes of calmodulin. We demonstrated that calmodulin continuously associates with the IQ1 during that calcium transition and that the IQ1 binding substantially changes the thermodynamic and kinetics of calcium transition in calmodulin. These findings provide insight into the mechanism by which calcium regulates myosin-5a.