Research and development on immature durian fruits grading machine consisted of two research experiment. Firstly, study and development on non-destructive maturity and internal quality determination machine by natural frequency value of fruit. Fast Fourier Transform attained from MatLab, high-level programming language and interactive environment that able to perform computationally intensive tasks. Quality grading machine consist of fruit conveyor, knocking tip and controlling circuit, sonic receiver, signal amplifier and conditioning circuit, signal controlling circuit and microcomputer with MatLab program. Prototype grading machine is able to determine non- destructively of at least 2,000 fruits per hour within an error of ± 2.5 % Secondary, study and development on non-destructive maturity and internal quality determination by dry matter of flesh fruits from Eastern and Southern parts of Thailand were done for high accuracy and easy procedure in under-matured separation of durian. Objective test in terms of digital value would be applied as standard for quality grading of Thai durians. Two durian electrical parameters such as resistance (R) and capacitance (C) were measured by standard meter are significantly related with their dry matter (DM). Exponential expressions of quality determination of durian could be utilized as database in prototype microcontroller grading machine design and development. Wide spread, precise and speedy prototype grading machine would be achieved to decrease immature combination.