In order to understand the physiological basis of a light green stripe mutant of Oncidium ‘Milliongolds’,we investigated the changes of the photosynthetic pigments and chlorophyll synthesis precursors contents,ultrastructure of chloroplasts and chlorophyll fluorescence parameters.The results showed that:(1)the contents of chlorophyll a (Chl a),chlorophyll b (Chl b),carotenoids (Car) and total chlorophyll (Chl)in mutant leaves revealed a remarkably decreased tendency,to decreased by 37.1%,34.0%,30.8% and 36.3% respectively (P<0.05).(2) The biosynthesis of chlorophyll in mutant was blocked at the reaction from prophobilinogen (PBG) to uroporphyrinogen (UrogenⅢ).(3) In mutant leaves,the number of chloroplast reduced and there were some abnormal chloroplasts with less grana lamellas,as well as more osmioplilic granules and vesicae.(4) The minimal fluorescence (F_o) of mutant was 39% higher than that of normal plant.Compared with normal plant,the maximal fluorescence (F_m),maximum quantum yield of PSⅡ(F_v/F_m),photochemical efficiency of PSⅡ in the light (F_v′/F_m′) and actual photochemical efficiency of PSⅡ (Φ_PSⅡ) reduced significantlyin mutant,while the photochemical quenching (qP) and non photochemical quenching coefficient (NPQ) of mutant were not significantly different from that of normal plant.It was concluded that the blockage of chlorophyll biosynthesis and aberrant chloroplast lead to the decrease of chlorophyll contents,light green stripe of leaves and lightharvesting capacity in mutant.