This experiment was conducted to evaluate the effects of expanded corn additive proportion under the secondary pelleting process on the processing quality of pellet feed and growth performance, nutrient apparent digestibility and serum biochemical indexes of weaned piglets. Five expanded corn additive proportions (the percentage of additive amount of expanded corn to total additive amount of corn) were designed in diets which were 0, 25%, 50%, 75% and 100%, respectively, and the diets were made into pellet feed by secondary pelleting process. Eighty 28-day-old weaned piglets with an initial body weight of (8.57±0.87) kg were randomly divided into 5 groups with 4 replicates per group and 4 weaned piglets per replicate. The experiment lasted for 50 days, and it was divided into 2 stages:early stage (the 1st to 28th day) and later stage (the 29th to 50th day). The results showed as follows:1) in the pellet feed processing quality, the pellet feed hardness, pellet durability index (PDI) and starch gelatinization degree were increased with the additive proportion of extruded corn increasing. 2) In the growth performance of weaned piglets, in early period, there were no significant differences among groups in the final body weight, average body weight (ADG) and average daily feed intake (ADFI) (P>0.05), but the feed conversion rate (FCR) in no-added group was significantly higher than that in other groups (P<0.05); in later period, there were no significant differences among groups in the final weight, ADG, ADFI and FCR (P>0.05); in full period, there were no significant differences among groups in the final weight, ADG, ADFI and FCR (P>0.05). 3) In the nutrient apparent digestibility of weaned piglets, the dry matter apparent digestibility and crude protein apparent digestibility in 100% group had the highest values and in no-added group had the lowest values, and they in no-added group were significantly lower than those in added expanded corn groups (P<0.05). 4) In the serum biochemical indexes of weaned piglets, the serum total protein content in 75% group was significantly higher than that in no-added group, 50% group and 100% group (P<0.05); the serum triglyceride content in no-added group was significantly higher than that in 50% group, 75% group and 100% group (P<0.05); the serum alkaline phosphatase (ALP) activity in 100% group was significantly higher than that in no-added group (P<0.05); the serum calcium content in 50% group was significantly lower than that in other groups (P<0.05). Considering the effects of the additive proportion of expanded corn on the pellet feed hardness, the growth performance of weaned piglets and the processing cost, it is suggested that the additive amount of expanded corn in the diet of weaned piglets is 25% of the total additive amount of corn under the secondary pelleting process.