Six different Japanese Maples (Acer palmatum) cultivars `Water Fall', `Burgundy Lace', `Crimson Queen', `Oshio-Beni', `SangoKaKu', and `Bloodgood' from Monrovia Nursery were planted in a randomized block design on 4 June 1997 at the The Morton Arboretum. Leaf heat tolerance was evaluated by measuring ion leakage of the leaf tissue at 25–60 °C in July, Aug., and Sept. 1997. The LT50 (the temperature at which 50% of the tissues were injured) of all the cultivars were higher in July (≈53 °C) and were lower in September (≈47 °C). Water content of the leaf tissues were higher in July compare to August and September and were not related to heat tolerance of most cultivars. Stem cold hardiness was performed by artificial freezing tests in Oct., Dec., and Feb. 1997/98. The Lowest Survival Temperature (LST) for the most hardy to least hardy cultivars in October and December were: `Burgundy Lace' (–15, –27 °C), `Bloodgood' (–18, –24 °C), `Oshio-Beni' (–15, –24 °C), `Crimson Queen' (–15, –18 °C), `Water Fall' (–9, –18 °C) and `SangoKaKu' (–9, –12 °C), respectively. Growth, dormancy development, spring budbreak and performance of these cultivars will be compared.