The aim of this study was to investigate the effects of in-ovo L-carnitine injection on hatchability of Domyati duck eggs and posthatching growth performance of ducklings. A total of 540 eggs were randomly divided into six treatments group. The 1^st, 2^nd, and 3^rd groups were: negative control (C); sham-injected control (P) and solution injected (S) groups, while, the 4^th, 5^th, and 6^th groups were in-ovo injected with 0.1 ml/egg saline solution containing 10, 20 or 30 mg of L-car, respectively. A total number of 180 ducklings hatched from the six experimental treatment groups were weighed and distributed into six experimental groups (30 ducklings each) and reared during growth period (0-12 wks of age) to study the effect of in-ovo experimental treatments on growth performance, carcass traits, blood plasma parameters and breast muscle histology.  The obtained results could be summarized as follows: In-ovo L-car (10 mg) injection had significantly (p≤0.01) the best value of hatchability (%) followed by control groups. All L-car doses (10, 20, and 30 mg) recorded high significant (p≤0.01) values for live body weight from hatch up to the end of experimental period, with the low dose (10 mg) being the best. The body weight gain, and feed conversion ratio were significantly increased, while, feed consumption was decreased in ducklings from L-car treated eggs during the overall periods compared to the control groups. Carcass traits for ducklings from all L-car treated eggs were significantly (p≤0.01) improved while, decreased in abdominal fat (%) as compared to control groups. Plasma HDL was significantly (p≤0.05) increased and LDL decreased in ducklings from in-ovo L-car injection treatments as compared with the control groups. Plasma triiodothyronine (T3) and Insulin-like growth factot-1 concentration were significantly (p≤0.01) higher for the in-ovo L-car injected groups than the control groups. Histological observations showed pronounced increases in myofibrils size and number in breast muscle sections of L-car ducklings compare to the control ones; the 10 mg injected group was the best.  It is concluded that in-ovo injection of 10 mg L-car in Domyati duck eggs at day 17 of incubation period had positive effects on hatchability, blood plasma constituents, IGF-1, T3 hormone, growth performance and carcass traits during summer condition in Egypt.