diff --git a/csst_mci_sim/csst_mci_sim.py b/csst_mci_sim/csst_mci_sim.py
index 61abc7e879394168c937c666744c60e32da0ea2c..8eed5b53b8d6be9b7c3bca08da40c212c3a9eb9f 100644
--- a/csst_mci_sim/csst_mci_sim.py
+++ b/csst_mci_sim/csst_mci_sim.py
@@ -2749,8 +2749,8 @@ class MCIsimulator():
                     photons=galsim.PhotonArray.makeFromImage(stamp_img, max_flux=1.0)
                     
                     cx0,cy0=centroid(stamp_img.array)
-                    ########### apply fat effect #####    
-                    if self.appFatt =='yes': 
+                    ########### apply fat effect to galaxy image #####    
+                    if self.appFatt : 
                         
                     ### apply treering and bright fatter and diffusion;