According to the classical crystallization theory, the influences of kinetics and thermodynamics on glassforming ability （GFA） for bulk metallic glasses has been investigated. The theoretical analysis shows that crystallization resistance is in proportion to the viscosity of “nose” temperature （Tn） while crystallization driving force is inversely proportional to the viscosity of crystallization onset temperature （Tx） on reheating in timetemperaturetransformation （TTT） curve, and therefore a GFA parameter ω0, defined as （Tg-T0）/（Tx-T0）-（Tg-T0）/（Tn-T0）, was proposed （wherein Tg and T0 are glass transition temperature and Vogel temperature respectively）. The parameter ω0 shows an excellent correlation with the critical cooling rate for glass formation of bulk metallic glasses, with the statistical correlation factor of R2=09626. Furthermore, the relationships between the GFA and Trg, ΔTx, γ, γm, ΔTrg, α, β, δ and φ, as well as the fragility of liquid could be explained reasonably based on our proposed ω0 parameter.