Vegetation Indices（VIs） derived from remotely sensed data have been developed to monitor the Earth’s vegetation cover.However

the topographic influence on VIs is an inevitable issue and is usually neglected in their large scale applications.In this study

the topographic effects on four commonly used vegetation indices

including Simple Ratio（SR）

Normalized Difference Vegetation Index（NDVI）

Reduced Simple Ratio（RSR）

and Modified Normalized Difference Vegetation Index（MNDVI）

derived from Landsat TM data over a mountainous forest area are evaluated.Two simple methods

the cosine correction and C-correction models

with different treatments of the influence of the diffused irradiance on reflectance

are used to remove the topographic effects on selected VIs.The results indicate that the reflectance in the Near Infrared（NIR） and Short Wave Infrared（SWIR） bands are more sensitive to topographical variations than that in the red band.Diffused radiance from the sky in the red band can moderate the variations of red band reflectance with topography

while this moderation is weak in the NIR and SWIR bands.The topography affects strongly vegetation indices which are not expressed as band ratios

such as RSR and MNDVI

resulting in negative biases on Sun-facing slopes and positive biases on Sun-backing slopes.As the slope increases

these biases increase rapidly.Therefore

the topographic effects should be carefully removed before using these non-band-ratio vegetation indices for vegetation parameter retrieval.Vegetation indices which are expressed as band ratios

such as SR

NDVI

can greatly reduce the noise caused by topographical variations.However

these indices still include significant topographic effects on steep slopes.SR is more sensitive to topographical variations on steep slopes than NDVI.The C-correction model is much better than the cosine correction model in removing topographic effects on VIs

especially on steep slopes.