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This letter proposes a new type of microstrip slow-wave structure. The proposed structure has the signal strips and the inserted ground strips periodically loaded in the internal part of the conventional microstrip line. The proposed microstrip line has not only the same outline contour but also a lower phase velocity compared to the conventional one, especially for the thick substrate. In addition, the slow-wave factor and the characteristic impedance are easily varied by adjusting the structural parameters. A third-order Chebyshev filter with a passband ripple of 0.05 dB and a bandwidth of 10% was designed and fabricated using the proposed microstrip line. The simulated and measured results agree well with each other.