This study investigated the possibility of an injectable bone substitute consisting of methylcellulose aqueous solutions and poly lactide-co-glycolide acid/hydroxyapatite (PLGA/HA) composite microspheres containing sustained releasing antibiotics. HA nanoparticles were synthesized by coprecipitation. PLGA/HA composite microspheres were prepared by oil-in-water emulsion/solvent evaporation with tetracycline. They were mixed with a 2% (w/v) aqueous solution of methylcellulose. The particle size of the composite microspheres increased linearly with increasing PLGA concentration from 17.8±4.5 to 185.4±29.5 μm. As the size of PLGA/HA composite microspheres increased, the content of tetracycline increased from 0.8% to 70.3%. The release rate of tetracycline decreased with increasing size of the composite microspheres. Tetracycline exhibited a burst release with nearly 100% escape within 24 hrs when the PLGA concentration was less than 20%. Sustained continuous release until 2 weeks when PLGA content was 20%. The gelation temperature of methylcellulose can be adjusted to normal body temperature when NaCl content exceeds 5%. PLGA/HA composite microspheres mixed with a methylcellulose aqueous solution was easily and completely ejected through an 18G needle when the mixing ratio was below 100%.