Attempts were made to form regular arrays of size-and position-controlled Pt-dots on GaAs and InP by combining an in situ electrochemical process with the electron beam (EB) lithography. This utilizes the precipitation of Pt nano-particles at the initial stage of electrodeposition. First, electrochemical conditions were optimized in the mode of self-assembled dot array formation on unpatterned substrates. Minimum in-plane dot diameters of 22 nm and 26 nm on GaAs and InP, respectively, were obtained under the optimal pulsed mode. Then, Pt dots were selectively formed on patterned substrates with open circular windows formed by EB lithography, thereby realizing dot-position control. The Pt dot was found to have been deposited at the center of each open window, and the in-plane diameter of the dot could be controlled by the number, width and period of the pulse-waveform applied to substrates. A minimum diameter of 20 nm was realized in windows with a diameter of 100 nm, using a single pulse. Current-voltage (I-V) measurements using an atomic force microscopy (AFM) system with a conductive probe indicated that each Pt dot/n-GaAs contact possessed a high Schottky barrier height of about 1 eV.