  High fidelity two-qubit gates are fundamental for scaling up the
superconducting number. We use two qubits coupled via a frequency-tunable
coupler which can adjust the coupling strength, and demonstrate the CZ gate
using two different schemes, adiabatic and diabatic methods. The Clifford based
Randomized Benchmarking (RB) method is used to assess and optimize the CZ gate
fidelity. The fidelity of adiabatic and diabatic CZ gates are 99.53(8)% and
98.72(2)%, respectively. We also analyze the errors induced by the decoherence.
Comparing to 30 ns duration time of adiabatic CZ gate, the duration time of
diabatic CZ gate is 19 ns, revealing lower incoherence error rate
$r'_{\rm{incoherent, int}}$ = 0.0197(5) than $r_{\rm{incoherent, int}}$ =
0.0223(3).
