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Multichip LED arrays are widely used for lighting to provide high luminance. Luminous efficacy, lifetime, and color temperature are highly dependent on the temperature at p-n junction. This paper investigated the effects of distance, number of chips, and driving current on the thermal resistance of LED module. Thermal resistance dramatically increased as the distance between LED chips decreased due to significant thermal spreading impedance for heat dissipation from junction to ambient. The parallel-resistance formula substantially underestimated the junction temperature of the LED modules due to significant thermal crowding effect. Thermal boundary can also rise junction temperature as the distance to the board edge decreased in both the two-chip and four-chip modules. Infrared results showed that chip temperatures were highly consistent with thermal resistance measurements under different driving currents.