Weak leptonic decays of light and heavy pseudoscalar mesons are studied in a field-theoretic quark model where the mesons are described by constituent quarks and antiquarks with a particular ansatz and a Harmonic oscillator wave function. With constituent quark masses and Harmonic oscillator radii for mesons as parameters of the model, decay constants are calculated and found as (fπ, fk, fD, fB) = (132 MeV , 161 MeV , 160 MeV , 115 MeV ) indicating a reasonable agreement with the experimental data wherever available. Partial decay widths and branching ratios for kinematically allowed leptonic decay processes are also estimated and compared with the experiments wherever available.