Phenotypic plasticity represents a crucial mechanism by which organisms adapt to environmental changes, yet different environmental pressures may place opposing demands on fish physiological functions. Understanding how diurnal differences interact with predator effects to influence fish phenotypic traits is thus intriguing and essential. Fish metabolic and behavioral phenotypes, which are important components of their eco-physiological functional traits, are susceptible to diurnal physicochemical environmental changes, such as water temperature fluctuations, and also sensitive to biological environmental changes, such as changes in feeding or predatory pressure. Fish physiology tends to be down-regulated at night due to lower temperatures compared to daytime; however, for some prey fish, increased predation pressure at night places higher demands on physiological functions. In this study, a common garden experiment was conducted for 6 months with a blank control group, a non-predator control group, and a predator treatment group, using Rhodeus ocellatus, a widely distributed small fish in freshwater ecosystems, as the prey fish, and Channa argus as the predator and Carassius auratus as the non-predator. After that, the metabolic characteristics, including routine metabolic rate (RMRrout), maximal metabolic rate (MMR), metabolic scope (MS), and the anaerobic performance, including exhaustive exercise time, excess post-exercise oxygen consumption (EPOC), EPOC recovery duration, EPOC consumption, as well as the spontaneous behavior and chemical alarm responses of R. ocellatus were measured under diurnal and nocturnal conditions, respectively. The results showed that: (1) Both predator, day-night differences and their interactions had significant effects on RMRrout (P<0.05) and non-significant effects on MMR (P>0.05) of R. ocellatus. Besides, diurnal differences significantly affected MS (P<0.05) whereas predator treatment had no effect on MS (P>0.05); (2) Day-night differences significantly impacted EPOC recovery duration and EPOC consumption (P<0.05), while predator effects eliminated the day-night differences in EPOC consumption (P>0.05); (3) Both predator and day-night differences significantly influenced bottom-dwelling time and the time spent on surface (P<0.05), with predator effects reducing nocturnal bottom-dwelling time and increasing diurnal time spent on surface (P<0.05); (4) Predator treatment imposed significant influence on the chemical alarm responses of R. ocellatus (P<0.05) whereas day-night differences had no effect on the chemical alarm responses. The risky chemical cues led to a significant increase in motionless time of R. ocellatus under all treatments (P<0.05). The results suggest that the eco-physiological functional traits of R. ocellatus have remarkable diurnal variation, which can be enhanced or eliminated by predator effects. We highlighted that the metabolic and behavioral phenotypes of R. ocellatus are highly ecologically plastic and susceptible to changes in community environments, such as diurnal variation and predator effects.