Previous studies showed that Schizochytrium sp. (SS), a marine microalga, has positive effects on the growth and immunity of aquatic animals and poultry. This study aimed to determine the effect of SS supplementation on the growth performance and health conditions of dairy calves. Fifty newborn female Holstein calves were randomly assigned to 1 of 5 groups (n = 10). All calves were offered 4 L colostrum within 1 h after birth, and then 2 and 1 L colostrum 6 h and 18 h, respectively, after birth. The 5 groups of calves were fed milk supplemented with (per calf per d) 0 g (control, SS0), 5 g (SS5), 10 g (SS10), 20 g (SS20), or 40 g (SS40) of Schizochytrium sp. powder from d 4 to d 60. The supplementations ended at d 60 when the calves were weaned. The daily supplementations of SS were equally divided and fed twice at 0800 and 1500. During the experiment, DMI and fecal score were recorded daily, while bodyweight (BW), chest circumference, and body length were measured biweekly at d 1, 14, 28, 42, and 60. Data were analyzed using the Proc Mixed of SAS, with linear and quadratic polynomial contrasts tested using the CONTRAST statement. The appropriate coefficients for the CONTRAST statement of this study were obtained using PROC IML's ORPOL function. The results showed thatSShad no effect on the BW, chest circumference, body length, or DMI. However, the average daily gain (ADG) increased linearly with the increased SS supplementation (P < 0.05), with the ADG of SS20 being 16.76% and 16.33% higher than that of SS0 and SS5, respectively (P < 0.05). Feed-to-gain (F:G) ratio tended to linearly decreased (P < 0.05) with the increased SS supplementation, and the F:G were 12.18%, 15.91% and 9.8% lower in SS10, SS20, and SS40, respectively, than in SS0 (P < 0.05). From d 29 to d 42, fecal scores were quadratically changed with the increased SS supplementation, with SS20 displaying a parabolic nadir. The fecal score of SS20 and SS40 was 8.2% and 13%, respectively, lower than that of SS0. These results indicate thatSS, when added at an appropriate level (e.g., 20 g/d), may improve the growth performance and feed efficiency of dairy calves before weaning.