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By using the technique of electromagnetically induced transparency(EIT), we have observed the quantum interference phenomenon in the lower-lying atomic Rydberg states of cold Cs atom in a magneto-optical trap (MOT) with typical atom numbers up to $1\times 107$, and cloud temperature about 100$\mu$K. In our experiments, a grating feedback diode laser is frequency stabilized on the Cs atom via Doppler-free saturation absorption transition $6s2S1/2,F=4>\to 6p2P3/2,F\prime =5>$, and a tunable Ti:Sapphire laser scans across the $6p2P3/2,F\prime =5>\to 8s2S1/2,F\prime \prime =4>$ transition frequency. Both spontaneous emission fluorescence from MOT and transmission intensity of the diode laser are monitored. A subnatural linewidth signal is obtained for probing the transition of Cs $6p2P3/2,F\prime =5>\to 8s2S1/2,F\prime \prime =4>$. These signals correspond to the cascade EIT among the$6s2S1/2,F=4>\leftrightarrow 6p2P3/2,F\prime =5>\leftrightarrow 8s2S1/2,F\prime \prime =4>$. \vspace{1em} This project is financially sponsored by National Science Council (grand no. NSC-95-2112-M-012-MY3). \vspace{5em}