This study was conducted to determine the effects of supplemental LP20 on growth performance and immune response of postlarvae (PL) and juvenile stages of white leg shrimp, Litopenaeus vannamei. The LP20 contains 20% heat-killed Lactobacillus plantarum strain L-137 (HK L-137) and 80% dextrin in dried-weight basis. The study was conducted for two phases: Phase 1 (hatchery stage- PL₁ to PL₁₅) was carried out indoor with four treatments by supplementation of LP20 at 0.0 g (marked as LP20-0.0), 0.5 g (LP20-0.5) and 1.0 g (LP20-1.0) LP20 kg^-1 of feed to the basal diet. The basal diet without LP20 was set up as a control that used in the whole experimental period (including Phase 1 and Phase 2). Initial PLs of L. vannamei were cultured at stocking density of 75,000 nauplii 0.5 m³ tank^-1. The results showed that the final weight and length of PLs (at PL₁₅) fed feed containing 1.0 g of LP20 kg^-1 of feed were significantly higher than those of PLs fed basal diet (P). Total bacteria count in PL₁₅ was significantly different between LP20 supplemented groups (LP20-0.5 and LP20-1.0) and without LP20 supplemented groups (LP20-0.0 and control). Phase 2 (nursery stage- PL₁₆ to PL₄₅) was conducted continuously from Phase 1, in which, similar-sized shrimps of each treatment, including LP20-0.0, LP20-0.5 and LP20-1.0 were selected and divided into two groups, fed with a reduction of bacterial doses (at 0.1 g and 0.25 g kg^-1 of feed). The results showed that the highest specific growth rate (SGR) was found in the treatment LP20-1.0 that fed diet supplemented LP20 at 0.25 g kg^-1 of feed (1.0LP20-0.25) and significantly differed compared with the control (P); however, there was no significant difference in SGR among the LP20 supplemented diets. The highest bacterial densities were observed in the hepatopancreas of shrimp of the 0.5LP20-0.1 treatment (in comparison with other treatments, P). Similarly, the lowest Vibrio spp. density (10×10³ colony-forming units (cfu) sample^-1) was counted in the hepatopancreas of shrimp of the 0.5LP20-0.1 treatment. Bacterial challenge test showed that the significantly lowest cumulative mortality (7.5%) was found in the 0.5LP20-0.1 treatment when compared with other treatments (P). Thus, the findings of this study indicated that the supplementation of 0.5 g of LP20 kg^-1 of feed to the diet used for white leg shrimp at PL stage and continuing feeding PL with 0.1 g of LP20 kg^-1 of feed enhanced growth performance and improved immune system.