We report an experimental investigation of secondary reactions of intermediates in delayed coking. Thermal cracking reactions of intermediates, for example coker naphtha (C5 ~180 °C), light coker gas oil (LCGO, 180–350 °C), middle coker gas oil (MCGO, 350–440 °C), and heavy coker gas oil (HCGO, >440 °C), were investigated. The results reveal that cracking of coker naphtha and LCGO is low under these experimental conditions. Thermal cracking MCGO exceeds that of LCGO. Among all the intermediates, thermal cracking is greatest for HCGO. The secondary reactions of HCGO produce not only gas and liquid products, but also coke. This increase in the yields of gas and coke is attributed to secondary reactions of HCGO and MCGO. Inhibition of the secondary reactions of intermediates results in a greater yield of liquid.