High performance liquid chromatography (HPLC) methods for determination of organosulfur compounds (OSCs) before their enzymatic decomposition and allicin in garlic were optimized and validated. Four ${\gamma}$-glutamyl peptides, 2 S-alk(en)yl cysteine sulfoxides, and 2 Salk(en)yl cysteine were simply extracted with water, followed by the sensitive and selective determination of all compounds in a single run using reverse phase HPLC. Allicin was extracted with dichloromethane and diethyl ether and analyzed by normal phase HPLC/UV system. OSCs and allicin were separated on Hypurity Elite $C_{18}$ and silica gel column, respectively. These methods exhibited excellent linearity for all of the analytes with good correlation coefficient (r>0.984). Average recoveries were in the range of 81.89-102.38% with good standard deviations. The limits of quantitation (LOQs) of OSCs and allicin ranged from 0.76 to 5.79 and 20.09 ${\mu}g$/mL, respectively. The precision results were expressed as relative standard deviation (RSD), ranged from 0.03 to 3.31% for intra-day and from 1.22 to 14.00% for the interday. These analytical methods and validation data may be useful for quality evaluation of garlic bulbs.