LED have the special character that LED break out radiant power and heat when it is operating. Recently photo efficiency of LED is just 20% and almost residual power converts heat. But heat causes bad reliability and changes of electrical and optical character negatively. Therefore, the LED package design is the first issue for the solution of the heat problem. This study was to design a LED T-junction measurement platform which based on the required of JEDEC-51-1. The experiment results include four parts: (1) Manually retrieve V Fss voltage, the temperature raised around 3∼5°C between the chip junction temperature and LED pin; (2) Automatically retrieve V Fss voltage, the temperature raised around 30∼50°C for the same condition; (3) From trend of T J vs. time, the definition of the steady state require at least 900 sec, but we would suggest 1200 sec in this study; (4) There is no significant difference of the T J for the retrieval time 100 μs and 10μs. The important achievement of the results of this experiment can be used as a reference for T J measurement.