The present experiment was aimed to investigate the infestation of helminth parasites and their effects on different size groups of walking catfish, Clarias batrachus from haor basin of Sylhet region, Bangladesh. A total of 100 C. batrachus fish were collected from different natural water bodies like beels, haor and adjacent fish markets of Sylhet namely Lal Bazar, Major Tilla and Kazir Bazar. The collected C. batrachus were divided into three length groups namely small (<13cm), medium (13-18 cm) and larger (>18cm). Among thecollected 100 individuals of C. batrachus, 76 were found to be infested with 1855parasites individuals of seven different endohelminth parasitic species i.e. Orientocreadium batrachoides, Bovienia serialis, Lytocestusindicus, Lytocestus birmanicus, Djombangia penetrans, Pseudocaryophyllaeus indica and Paracamallanusspiculo gobernaculus all of which are belonging to three different groups (i.e. trematode, cestode and nematode). It reveals thatthe % gain in mean head length (GHL) and % weight loss (WL)of C. batrachus were found to be variedincreasingly in every length groups with high (snowballing) level of endohelminth infestation. Among the endo-helminth infested C. batrachus, comparatively higher % GHL6.0 were recorded in medium length group over smaller (5.0 cm) and larger group (5.0 cm), whereas higher mean % WL8.0 were recorded in smaller length group followed by larger (7.0) and medium (6.0) group. Seasonal variation in hleminths parasitic infestation and its effect explicated in-terms of % GHL and % WLin different length groups of C. batrachus revealed that high %GHL of C. batrachus along with % WL were observed in highly infested cluster over the low infested and non-infested set in every length group duringdifferent seasons of the yearnamely pre-monsoon, monsoon and post- monsoon.The observed highest % WL were recorded in smaller (13.0) length group in pre-monsoon and medium (14.0) group in monsoon, whereas lowest (1.0) in low infestation sub cluster of the larger length group wasrecorded during monsoon. The observed %GHL of C. batrachus were highest in highly infested cluster of smaller length group (12.0) and lowest in larger (2.0) group du ring pre-monsoon, followed by highest (11.0) in small and lowest (4.0) in medium length group during monsoon season.Thus the present finding reveals that over the different season of the year, % gain of mean head length (cm) and % WL (g) has increasing along with increased level of helminthes infestation.