Quantum memory is a core component of a quantum computer where quantum mechanics, instead classical mechanics, becomes dominant if the device size approaches nanometer scale. Because of it potential scalability and its long relaxation time T1, an electron spin in a semiconductor quantum dot is one of the most promising candidate for a solid-state-based quantum computer architecture. Decoherence of the electron spin in quantum dots, through hyperfine coupling to its surrounding nuclear spins, is the major obstacle for the realization of a quantum memory. Typical decoherence time T2* is 10 ns for GaAs quantum dots at low temperature (~100 mK) and in sub-Tesla magnetic field. I address the decoherence mechanism and the decoherence process of an electron spin in quantum dots, as well as quantum control of the decoherence to extend the coherence time 1000 times longer via dynamical decoupling method by subjecting the electron spin to a sequence of control pulses. Experimental related considerations, such as external magnetic field, initial bath polarization, and control pulse imperfection, are also discussed.