In the present investigation, a series of schiff base (1-20), carbohydrazide (31-43) and amides/anilides (21-30/44-56) of nalidixic acid was synthesized in appreciable yield and characterized by physicochemical as well as spectral means. The synthesized compounds were evaluated in vitro for their antimicrobial activity against Gram-positive bacteria S. aureus, B. subtilis, Gram negative bacteria E. coli & fungal strains C. albicans & A. niger by tube dilution method. Antimicrobial activity results indicated that compounds 16, 24, 36, 53 (MIC = 6.25 μg/mL) were the most potent antimicrobial agents. The results of antimicrobial screening also indicated that compounds having NO₂, Br, F, Cl, OCH₃ substituents were the most active one. mt-QSAR investigation used Hansch analysis was applied to find out correlation between antimicrobial activities with physicochemical properties of synthesized compounds. The mt-QSAR revealed that the antibacterial and antifungal activity of these synthesized derivatives against microorganisms under test was mainly governed by second order molecular connectivity index (²c)and zero order molecular connectivity index (⁰c^v). It also indicated the importance of zero order molecular connectivity index (⁰c^v)in describing the antimicrobial activity of synthesized compounds.