The German cockroach, Blattella germanica (L.), a prominent pest that requires management due to its detrimental economic and medical consequences. Several research discovered that German cockroaches were insecticide resistant, mainly commercial pesticides. One-week-old nymphs from two strains in Penang, Malaysia (Georgetown strain: EL and Greenlane strain: IC strain) were tested in the laboratory against two commercial insecticides, Cislin® 25 (deltamethrin) and Sumithion 50 (fenitrothion). The concentration of solutions used in the residual test based on the manufacturer labeling. Cislin® was tested at 1.90 ppm, 1.60 ppm, 1.30 ppm, and 1.0 ppm; Sumithion 50 was tested at 25 and 27.00 ppm, 23.00 ppm, 18.00 ppm, 14.00 ppm using the residual method. Probit analysis and one-way analysis of variance (ANOVA) were used to compare the data. Cislin® 25 and Sumithion 50 were more effective and high toxicity against the IC strain compared with the EL strain. Sumithion 50 demonstrated a fast knockdown time on cockroaches, but Cislin® 25 showed no knockdown time. Sumithion 50 showed a significant mortality rate in cockroaches within a short period of time compared to Cislin® 25. Both insecticides were found to be effective against both strains, but Sumithion 50 is more effective at controlling cockroaches than Cislin® 25.