Inclusion criteria:  at least 18 years of age or above able to understand the nature of the study and give informed consent, able to communicate in Cantonese, have a diagnosis of psychotic disorders (e.g., delusional/brief psychotic disorder, schizotypal disorder, and psychosis not otherwise specified) according to the DSM IV-TR, DSM-V or ICD-10, within 5 years of onset as documented in their clinical/written records; insufficiently active (i.e., <150 minutes of MVPA per week) based on self-reports in a brief screening form, and having used mobile instant-messaging apps (e.g. WhatsApp/Facebook messenger/WeChat) installed on a smartphone.  Exclusion criteria:  have had a record of alcohol abuse and/or substance misuse, poor physical health condition, acute/severe medical diseases seriously reducing their life expectancy or ability to participate, pregnancy or within six months of postpartum, have received (within the past 6 months) or are receiving other physical and psychosocial interventions, and severe psychiatric symptoms appeared in the past month.