A folded microstrip meander line (FMML) slow-wave structure (SWS) based on slotted dielectric substrate is proposed. Due to the metal meander line printed in a semi-circular slot of the dielectric substrate, the proposed FMML SWS is very suitable for traveling-wave tubes (TWT) with cylindrical electron beam (CEB), which can be focused by a traditional periodic permanent (PPM) magnetic focusing system. The analyses of dispersion characteristics, coupling impedances, transmission properties and particle-in-cell (PIC) of the proposed FMML SWS are investigated. Compared with traditional planar microstrip meander line (PMML) SWS, the FMML SWS has the advantages of lower phase velocities, weaker dispersion and higher coupling impedances, which make the FMML SWS suitable for developing low-voltage, wideband and miniature millimeter-wave TWTs. With a focus magnetic of only 0.4 T, a Ka band TWT with the FMML SWS is capable of delivering 42.32 W output power with a corresponding gain of 26.26 dB at 35 GHz, while the voltage and the current of the electron beam are set to be 6 550 V and 0.1 A, respectively.