Abstract: DNA sequences from rbc L and mat K genes were used to assess relationships between and within traditional Taxodiaceae and Cupressaceae (s. s. ) and together with fossil records to construct molecular clocks for estimating the divergence time of the major clades of Taxodiaceae and Cupressaceae (s. s. ). Relative-rate tests were used to assess homogeneity of substitution rates among the lineages. The genera or clades, which had not passed the test, were excluded from the molecular clock construction. The molecular clocks were calibrated using nonsynonymous substitution ratesper site and fossil records. The result of molecular clock analysis shows that the divergence time of major lineages of Taxodiaceae is in the Jurassic time, which supports the views that the modem Taxodiaceae were well established by the Jurassic time. The estimated time when the Southern and Northern Hemisphere clades of Cupressaceae (s. s. ) (Callitroideae and Cupressoideae) diverged is 124 Ma, which is consistent with the viewpoint that the two subfamilies formed with the separation of the two related ancient land masses (Gondwanaland and Laurasia) during the Late Jurassic or the Early Cretaceous, implying that the modern Cupressaceae (s. s. ) were well established by the Cretaceous time.

Key words: Cupressaceae (s.l.), Molecular phylogeny, Molecular clock, Fossil record, Divergence time