Summary form only given, as follows. A large area, permanent magnet ECR plasma source employing a buffer cavity and a TE01 mode converter has been designed and is being constructed at NTHU. The magnet structure consists of two sets of rings with radially directed magnets along with iron pole-pieces and a shielding envelope. Calculation using the PANDIRA code has been employed to yield the desired magnetic field profile. The 2.45 GHz microwave power is coupled from a rectangular waveguide to the TE10 circular waveguide mode through a Flower-Pedal mode converter. The design of the converter has been modeled by a finite element electromagnetic simulation code HFSS, yielding a conversion efficiency of 96%. A buffer cavity developed in our previous work is also employed to increase operation stability by reducing the variation of load impedance seen by the microwave generator. The ECR heating of plasma and wave propagation have been investigated by the PIC-MCC simulation code developed previously. Both the design and simulation results are presented.